[clang] [clang] [unittest] Add a test for Generic_GCC::GCCVersion::Parse (PR #69078)

[Alex Bradbury via cfe-commits]

asb wrote:

@tbaederr Just came to report the same thing!

@mstorsjo This broke builds that use `-DBUILD_SHARED_LIBS=True`. The problem seems to be that the `Generic_GCC` class has the `LLVM_LIBRARY_VISIBILITY` attribute meaning the `clang::driver::toolchains::Generic_GCC::GCCVersion::Parse(llvm::StringRef)` symbol is hidden. Removing that attribute fixes the build. It would be good for someone more familiar with this part of the codebase to confirm if that's an acceptable fix however.

https://github.com/llvm/llvm-project/pull/69078