[clang] [Cygwin] Export global symbols, template instantiations, RTTI and VTable are exported correctly (PR #138773)

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https://github.com/jeremyd2019 approved this pull request.

I think the changes themselves LGTM but I don't know if this is OK as one PR.  @mstorsjo ?

https://github.com/llvm/llvm-project/pull/138773