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

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jeremyd2019 wrote:

But people build mingw clang with gcc, right?  They presumably don't set `LLVM_ENABLE_PIC=OFF`.  It seems like having explicit dllexport annotations as libclang does should turn off auto-export.  I didn't see `--export-all-symbols` in the command line anyway.

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