-Wgcc-install-dir-libstdcxx is broken

Warnings about multiple gcc installations are issued even if clang driver got nothing to do with any gcc whatsoever (e.g., in combination with `-stdlib=libc++ --rtlib=compiler-rt --unwindlib=libunwind`). With -Wall -Werror it's very disruptive.

Observed on version: 22.1.2

Reproduce the warning with the reported non-GCC runtime combination, including -Wall -Werror, and compare it with a GCC-related invocation. Trace the -Wgcc-install-dir-libstdcxx diagnostic in the clang driver; done means unrelated libc++/compiler-rt/libunwind invocations no longer emit the warning.

Written by the indexing model from the issue text.