[clang] [Preprocessor] Do not expand macros if the input is already preprocessed (PR #137665)

[Juan Manuel Martinez Caamaño via cfe-commits]

jmmartinez wrote:

I fixed the `Modules/initializers.cpp` test by moving the preprocessor directives outside of the module contents (the module contents should be already preprocessed and not contain preproc directives).

https://github.com/llvm/llvm-project/pull/137665