[clang] [clang] Add support for MSVC force inline attrs (PR #185282)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > Thank you for working on this! Please be sure to add a release note to `clang/docs/ReleaseNotes.rst` so users know about the new functionality too.
> 
> @AaronBallman Thanks for the review. I added a point to the "Attribute Changes" section - not sure if this is the right place since `[[msvc::` attributes are rarely mentioned in the previous release notes.

That's a good place for it!

> > Are there plans to also handle `[[msvc::forceinline_calls]]` (https://learn.microsoft.com/en-us/cpp/cpp/attributes?view=msvc-170#msvcforceinline_calls) which sounds like our `[[clang::always_inline]]` behavior on statements?
> 
> I am aware of `[[msvc::forceinline_calls]]`. Initially I wanted to keep the PR simple, so I let `[[msvc::forceinline]]` have an "unspecified" effect when added to statements, as MS documentation did not specify what would happen in that case.

This is interesting; it seems cl silently accepts the attribute on non-declaration statements, but diagnoses any declaration other than a function declaration as `attribute [[msvc::forceinline]] may only be applied to a function declaration`: https://godbolt.org/z/daG9nr1qc

But it doesn't have any effect on the call behavior, at least that I can find: https://godbolt.org/z/cr9TKcx5M

We'd prefer to diagnose the attribute as being explicitly ignored so users are not confused (that's our usual position on all ignored attributes). But because Microsoft accepts, I think this is a bug that should be [reported to Microsoft](https://developercommunity.visualstudio.com/VisualStudio/) to see if they'll change behavior. Things get harder for users if we accept code Microsoft rejects or we reject code Microsoft accepts, so we try to honor the vendor's extensions.

> Now, with all the additional diagnostics requirements, supporting `[[msvc::forceinline_calls]]` comes at a relatively low cost, so I added that.

Thanks!


https://github.com/llvm/llvm-project/pull/185282