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

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

I like this approach a lot, in that in think the approach is workable.
However I'm concerned that it's too ad-hoc. We know we will want to preserve _MSC_VER too, for example (but also __INTRIN_H, maybe other)

can we do `#pragma clang define name value` ?

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