[clang] [Clang] Support libstdc++ workarounds when using `-E` (PR #210802)

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Sirraide wrote:

Ok, in that case should we generalise the spelling to something like `#pragma clang __set_pp_state` (adding the leading `__` to the pragma might help convey that it’s for internal use only) which would then take a name + value where currently the only such pair that we accept is `glibcxx_version` followed by an integer (the name doesn’t really have to be the same as the macro name since it doesn’t define the macro anyway).

https://github.com/llvm/llvm-project/pull/210802