[clang] [llvm] [AArch64] Add -mlr-for-calls-only to replace the now removed -ffixed-x30 flag. (PR #98073)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic approved this pull request.

The updated approach makes sense, I think.

Please check that AArch64TargetLowering::LowerRETURNADDR works correctly when LR is reserved; I think it should just work, but I'm not completely sure.

Otherwise LGTM

https://github.com/llvm/llvm-project/pull/98073