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

[Mateusz Mikuła via cfe-commits]

mati865 wrote:

> But people build mingw clang with gcc, right?

Dunno.

> 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.

Yes, linker exports all the symbols by default, but explicit export disable that.

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