[clang] [win][aarch64] Always reserve frame pointers for Arm64 Windows (PR #146582)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

>From an ABI perspective, this seems fine: if you want fp to always be valid, sure, lets enforce that.

Do we generate appropriate diagnostics for inline asm?

Does the AArch64 backend actually support this flag?  I briefly tried, and it looks like this doesn't actually do anything.  I think we only implemented support for 32-bit Arm.

Should we make some corresponding change to LLVM so frontends that don't explicitly request some specific frame-pointer behavior follow the ABI by default?

https://github.com/llvm/llvm-project/pull/146582