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

[Fangrui Song via cfe-commits]

MaskRay wrote:

I hope that we do not drop `LLVM_LIBRARY_VISIBILITY` arbitrarily from `clang::driver::toolchains::*` classes, just because some unittests need to reference the symbols in a shared object.

```c
#if !defined(LLVM_BUILD_SHARED_LIBS)
```

is not great but is not too bad. `-DBUILD_SHARED_LIBS=on` modes are slow to execute tests and are not used often for Release builds.

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