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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I'm not against having a security hardening feature to avoid using LR as a GPR.  But we'd want to name it something different, to make it clear that it's a security-hardening feature, not a guarantee that the compiler won't clobber LR.

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