[clang] [clang] [MinGW] Don't look for a GCC in path if the install base has a proper mingw sysroot (PR #76949)

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https://github.com/jeremyd2019 approved this pull request.

> Looks mostly good to me, but I wonder if we should change testTriple as well.

I thought so too based on the comment, but reviewing the code it seems `testTriple` is trying to find evidence that a given triple (and more specifically arch for things like `i386` vs `i686`) is valid.  The evidence found by `looksLikeMinGWSysroot` does not provide any hint about what the triple or arch name should be, so I don't think it helps `testTriple`.

https://github.com/llvm/llvm-project/pull/76949