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

[Juan Manuel Martinez Caamaño via cfe-commits]

jmmartinez wrote:

> This seems reasonable. Can you update the description to explain the motivation for the change, though? Is it a bug you are trying to fix, or a specific scenario you think should be optimized? Thanks

I've updated the description with an explanation of the situation that triggered the issue for us. But in the wider scheme, I think that no preprocessing the preprocessor's output is the actual desired behavior.

Thanks!

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