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

[Timm Baeder via cfe-commits]

tbaederr wrote:

This doesn't build for me locally:
```
mold: error: undefined symbol: tools/clang/unittests/Driver/CMakeFiles/ClangDriverTests.dir/GCCVersionTest.cpp.o: clang::driver::toolchains::Generic_GCC::GCCVersion::Parse(llvm::StringRef)
```

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