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

[Tom Stellard via cfe-commits]

tstellar wrote:

> When your system GCC moves 12 to 13, do you remove `/usr/lib/gcc/x86_64-linux-gnu/12` and add `/usr/lib/gcc/x86_64-linux-gnu/13` atomically?

Yes.

> Gentoo doesn't seem to have more driver option requirement. Perhaps @mgorny @thesamesam can talk about they upgrade GCC and update Clang configuration files.

Isn't this because the clang driver has custom logic for detecting gcc on Gentoo?

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