[clang] [llvm] [clang][Inliner] Add opt-in inliner hints and implementation-file functions (PR #228074)

[Nikita Popov via cfe-commits]

https://github.com/nikic commented:

LLVM side of this looks good to me. Clang side should be reviewed by someone else -- though I'll already note that Clang is not supposed to use `cl::opt` options...

>From the LLVM side, I think the only questions I have would be:

 * Should we verify that the attribute is only used with linkonce_odr linkage? Given that the attribute is non-semantic, I think not verifying is fine, and maybe preferable.
 * Should this be an enum attribute instead of a string attribute? Given that it's only check in this one place I'm fine with the string attribute, but making it an enum attribute is probably more principled.

https://github.com/llvm/llvm-project/pull/228074