[clang] [llvm] [llvm][Triple] Add `Environment` members and parsing for glibc/musl parity. (PR #107664)

[Alex Rønne Petersen via cfe-commits]

alexrp wrote:

I obviously can't speak to Rich's opinion on this, but I'll just note that [musl's `configure` script](https://git.musl-libc.org/cgit/musl/tree/configure) only cares about the arch component of the triple; it does not match on the environment at all.

My understanding is that all of these `musl*` environments have been added to GCC and LLVM on an as-needed basis for toolchains, not necessarily because musl itself needs them.

https://github.com/llvm/llvm-project/pull/107664