[llvm] [ARM64EC] Warn on using disallowed registers in assembly src. (PR #93618)

[Namish Kukreja via llvm-commits]

namikukr wrote:

> Thanks, this looks very valuable!
> 
> Can you mention how to enable this warning when using armasm64? (Or I guess it probably gets enabled automatically if assembling with some flag to switch to arm64ec mode - what's the armasm64 flag for that?)

When assembling with `cl` on Windows, the flag is `-arm64EC`. It's mirrored similarly by `clang-cl`. I'll include that in the commit message.

https://github.com/llvm/llvm-project/pull/93618