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

This adds actual test cases for all the cases that are listed in a code
comment in the implementation of this function; having such test
coverage eases doing further modifications to the function.

This relands b4b35a5d. This time,
the new test is excluded if building with dylibs or shared libraries
enabled, as the clang::toolchains::Generic_GCC class is marked
LLVM_LIBRARY_VISIBILITY, giving it hidden visibility in such builds,
making it unreferencable outside of the dylib/shared library.