[clang] [clang] Mark some language options as benign. (PR #131569)

[via cfe-commits]

cor3ntin wrote:

The issue is that if we allow the preprocessor's state to differ across modules, then the resulting compiled units can be arbitrary (and subtly incompatible).

I wonder if a possible solution is to record which macros are used (ie, are expanded or appear in `#ifdef` / `defined`)
And ONLY serialize the set of macros that are used.
That way, if your program never uses `__SSP__`, for example, then your module would not be incompatible just because __SPP__ has been defined.

I think that's worth pondering a bit more @Bigcheese 



https://github.com/llvm/llvm-project/pull/131569