[clang] [clang][Driver] Support passing arbitrary args to -cc1as with -Xclangas. (PR #100714)

[Alex Rønne Petersen via cfe-commits]

alexrp wrote:

@MaskRay ping on this, specifically my comments here:

* https://github.com/llvm/llvm-project/issues/97517#issuecomment-2253551448
* https://github.com/llvm/llvm-project/issues/97517#issuecomment-2253571085

We keep getting bug reports about target features not being passed to Clang for assembly files, and we don't really have a good response to give to users. I don't see a reasonable path forward that doesn't involve adding something like `-Xclangas` so we can pass the exhaustive list of target features along to the assembler, just as we do with `-Xclang` for C-family files today.

https://github.com/llvm/llvm-project/pull/100714