[clang] [Clang][MicrosoftMangle] Implement mangling for ConstantMatrixType (PR #134930)

[Reid Kleckner via cfe-commits]

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

Thanks, looks good to me!

I wanted to add llvm-cxxfilt to the test to confirm these all demnagle to something readable, but I see it doesn't work on Microsoft symbols today, which is unfortunate. The code I want is on github in @nico's https://github.com/nico/demumble/blob/main/demumble.cc sources, but we shouldn't block merging on that.

https://github.com/llvm/llvm-project/pull/134930