[clang] Make predefined expressions string literals under -fms-extensions

MSVC makes these string literals [1][2].

[1] https://godbolt.org/z/6vnTzbExx
[2] https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=msvc-170

Fixes #114

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D146764