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

Unlike the `-Xassembler` idea mentioned in #97517, this one comes with
no GCC compatibility concerns, and simply mirrors what `-Xclang` does
for `-cc1`.

This is useful for mostly the same reasons that `-Xclang` is. The
motivating use case is `zig cc`, however, where we use `-Xclang
-target-feature` to pass the exhaustive list of target features to Clang
for C-family files. Before this commit, there was no way to do the same
for assembly files.

For context, Zig bases all of its target feature info directly on the
info in LLVM's backends, including all the dependency relationships
therein. So it just makes more sense for Zig to be able to directly pass
all this info to the assembler, rather than having to manually
reconstruct the corresponding frontend command line flags for every
target.

Closes #97517.

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