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

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

Thanks, the changes look good to me. However, as this is already neatly split into three separate commits, it would be nice to retain that separation after merging. The llvm github repo is configured to only do "squash and merge", so to retain the separation, it would have to be filed in three separate PRs. If they don't touch any overlapping code it's easy; if not, it's perhaps easiest to file one at a time in this case, and I can try to merge it in a timely fashion.

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