Use windows baskslash on anonymous tag locations if using MSVCFormatting and...

Use windows baskslash on anonymous tag locations if using MSVCFormatting and it's not absolute path.

This fixes a nondeterminism on debug info when building on windows natively vs
cross building to windows.

[1] https://github.com/llvm/llvm-project/blob/llvmorg-17-init/clang/lib/Lex/HeaderSearch.cpp#L465

Differential Revision: https://reviews.llvm.org/D150817