[lldb][Windows] Enable cxx_interop backward stepping tests

[charles-zablit]

The backward interop tests put main() in C++ (main.cpp) and set a source breakpoint there. Their Makefile assignedCFLAGS = -I. -g, which overrode Makefile.rules' DEBUG_INFO_FLAG default, so on Windows clang emitted CodeView for main.cpp while the Swift side used DWARF.

This patch moves -I. into CFLAGS_EXTRAS so the default debug flag (-gdwarf on Windows) applies to the C++ main as well.

rdar://179241795

[charles-zablit]

@swift-ci please test

[charles-zablit]

any category of interest for this run) 
[2026-06-10T16:46:13.614Z] ======================================================================
[2026-06-10T16:46:13.614Z] FAIL: test_setter_step_in_class_dwarf_clang_swift (TestSwiftBackwardInteropSteppingClass.TestSwiftBackwardInteropSteppingClass)
[2026-06-10T16:46:13.614Z] ----------------------------------------------------------------------
[2026-06-10T16:46:13.614Z] Traceback (most recent call last):
[2026-06-10T16:46:13.614Z]   File "/home/build-user/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 1941, in variant_method
[2026-06-10T16:46:13.614Z]     return method(self)
[2026-06-10T16:46:13.614Z]   File "/home/build-user/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 1941, in variant_method
[2026-06-10T16:46:13.614Z]     return method(self)
[2026-06-10T16:46:13.614Z]   File "/home/build-user/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 2087, in test_method
[2026-06-10T16:46:13.614Z]     return attrvalue(self)
[2026-06-10T16:46:13.614Z]   File "/home/build-user/llvm-project/lldb/packages/Python/lldbsuite/test/decorators.py", line 154, in wrapper
[2026-06-10T16:46:13.614Z]     return func(*args, **kwargs)
[2026-06-10T16:46:13.614Z]   File "/home/build-user/llvm-project/lldb/test/API/lang/swift/cxx_interop/backward/stepping/TestSwiftBackwardInteropSteppingClass.py", line 87, in test_setter_step_in_class
[2026-06-10T16:46:13.614Z]     self.check_step_in(thread, 'testSetter', 'SwiftClass.swiftProperty.setter')
[2026-06-10T16:46:13.614Z]   File "/home/build-user/llvm-project/lldb/test/API/lang/swift/cxx_interop/backward/stepping/TestSwiftBackwardInteropSteppingClass.py", line 23, in check_step_in
[2026-06-10T16:46:13.614Z]     self.assertIn(callee, name)
[2026-06-10T16:46:13.614Z] AssertionError: 'SwiftClass.swiftProperty.setter' not found in 'testSetterClass()'
[2026-06-10T16:46:13.614Z] Config=x86_64-/home/build-user/build/Ninja-ReleaseAssert/llvm-linux-x86_64/bin/clang
[2026-06-10T16:46:13.614Z] ======================================================================
[2026-06-10T16:46:13.614Z] FAIL: test_setter_step_in_class_dwarf_noclang_swift (TestSwiftBackwardInteropSteppingClass.TestSwiftBackwardInteropSteppingClass)
[2026-06-10T16:46:13.614Z] ----------------------------------------------------------------------
[2026-06-10T16:46:13.614Z] Traceback (most recent call last):
[2026-06-10T16:46:13.614Z]   File "/home/build-user/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 1941, in variant_method
[2026-06-10T16:46:13.614Z]     return method(self)
[2026-06-10T16:46:13.614Z]   File "/home/build-user/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 1941, in variant_method
[2026-06-10T16:46:13.614Z]     return method(self)
[2026-06-10T16:46:13.614Z]   File "/home/build-user/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 2087, in test_method
[2026-06-10T16:46:13.614Z]     return attrvalue(self)
[2026-06-10T16:46:13.614Z]   File "/home/build-user/llvm-project/lldb/packages/Python/lldbsuite/test/decorators.py", line 154, in wrapper
[2026-06-10T16:46:13.614Z]     return func(*args, **kwargs)
[2026-06-10T16:46:13.614Z]   File "/home/build-user/llvm-project/lldb/test/API/lang/swift/cxx_interop/backward/stepping/TestSwiftBackwardInteropSteppingClass.py", line 87, in test_setter_step_in_class
[2026-06-10T16:46:13.614Z]     self.check_step_in(thread, 'testSetter', 'SwiftClass.swiftProperty.setter')
[2026-06-10T16:46:13.614Z]   File "/home/build-user/llvm-project/lldb/test/API/lang/swift/cxx_interop/backward/stepping/TestSwiftBackwardInteropSteppingClass.py", line 23, in check_step_in
[2026-06-10T16:46:13.614Z]     self.assertIn(callee, name)
[2026-06-10T16:46:13.614Z] AssertionError: 'SwiftClass.swiftProperty.setter' not found in 'testSetterClass()'
[2026-06-10T16:46:13.614Z] Config=x86_64-/home/build-user/build/Ninja-ReleaseAssert/llvm-linux-x86_64/bin/clang
[2026-06-10T16:46:13.614Z] ----------------------------------------------------------------------
[2026-06-10T16:46:13.614Z] Ran 108 tests in 53.158s
[2026-06-10T16:46:13.614Z] 
[2026-06-10T16:46:13.614Z] FAILED (failures=2, skipped=84)
[2026-06-10T16:46:13.614Z] 
[2026-06-10T16:46:13.614Z] --
[2026-06-10T16:46:13.614Z] 
[2026-06-10T16:46:13.614Z] ********************
[2026-06-10T16:46:21.698Z] Testing:  0.. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90.. 
[2026-06-10T16:46:21.698Z] ********************
[2026-06-10T16:46:21.698Z] Failed Tests (2):
[2026-06-10T16:46:21.698Z]   lldb-api :: lang/swift/cxx_interop/backward/stepping/TestSwiftBackwardInteropSteppingClass.py
[2026-06-10T16:46:21.698Z]   lldb-api :: lang/swift/cxx_interop/backward/stepping/TestSwiftBackwardInteropSteppingStruct.py

[charles-zablit]

@swift-ci please test

[charles-zablit]

swiftlang/swift#90041

@swift-ci please test