[llvm] Support for ARM64EC in LLVM 18

[dpaoliello]

This is a meta-issue to gather any fixes made to the main branch to enable backporting to the v18 release branch to enable support for ARM64EC in LLVM 18.

More information about ARM64EC: https://learn.microsoft.com/en-us/windows/arm/arm64ec

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: Daniel Paoliello (dpaoliello)

This is a meta-issue to gather any fixes made to the `main` branch to enable backporting to the v18 release branch to enable support for ARM64EC in LLVM 18.

[dpaoliello]

/cherry-pick c761b4a 8f07014

[llvmbot]

Failed to cherry-pick: 79774

https://github.com/llvm/llvm-project/actions/runs/7791740777

Please manually backport the fix and push it to your github fork. Once this is done, please create a pull request

[llvmbot]

/pull-request #80754