[clang] Driver: Accept the --gcc-install-dir option multiple times (PR #71446)

[Tom Stellard via cfe-commits]

tstellar wrote:

> I think I have expressed I wanted to say... The new probing semantics seems really unusual to me. I do not feel that how path-related options usually behave.
> 

@MaskRay Ok, what do you think would be a good alternative?  Another solution I considered was to have --gcc-install-dir accept paths like /usr/lib/gcc/x86_64-redhat-linux/ and then probe for the 'best' version in that directory, but I felt this complicated the logic too much.  Probably the most simple solution that would satisfy our requirements would be to add a new option `--gcc-triple` which would use the same probing logic as the driver does now except it would cause the driver to skip all the hard-coded triples and only search for the one specified by the option.

> Can the system be changed to keep both versions around before removing GCC 12?

No, it can't.

https://github.com/llvm/llvm-project/pull/71446