[clang] [llvm] Revert "[AArch64] Add support for -ffixed-x30" (PR #88019)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

We have a mechanism which specifically encodes the distinction we want here: ReserveXRegisterForRA.  This was implemented for debugging/regression tests (see https://reviews.llvm.org/D132717), but we can expose it more generally.

Naming is hard, but maybe `-mavoid-non-call-lr` is better.

https://github.com/llvm/llvm-project/pull/88019