[clang] [Clang][Cygwin] attempt to fix building shared libclang. (PR #138351)

[via cfe-commits]

jeremyd2019 wrote:

Surprisingly, it looks like Cygwin (@tyan0) is working around the export limit on gcc by setting `-DCMAKE_SHARED_LINKER_FLAGS=-fvisibility=hidden`.  I was surprised that worked on gcc - maybe it can be generalized here instead of fighting to build only static with gcc to then use that clang to build dynamic libraries?

https://github.com/llvm/llvm-project/pull/138351