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

[Amara Emerson via cfe-commits]

aemerson wrote:

@efriedma-quic @francisvm Can we restore this change? We need this for security hardening purposes to prevent LR allocated for general uses.

(There is also an issue with reserving LR in that AArch64's PEI doesn't add live-in flags for reserved regs, but I think we should fix that there once we restore this feature).

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