[clang] [llvm] [clang][LTO] Emit symbols for global inline assembly as module flags (PR #174995)

[Andrew Savonichev via cfe-commits]

asavonic wrote:

@mysterymath, @ilovepi, @efriedma-quic, thank you for review!

> Generally, I'd like to minimize the amount of code for LTO that's in clang/: we have other frontends that do LTO, and we don't want them to copy-paste/translate a bunch of new code just to make inline asm work the way it's supposed to. Can we refactor this?

Sure! Perhaps we can move `ModuleSymbolTable::CollectAsm*` calls and metadata construction to `ModuleSymbolTable` itself? Then Clang or other frontends can call `ModuleSymbolTable` to get a constructed metadata, and set it as a flag. Or let `ModuleSymbolTable` to update a Module.

> Can you add a RISC-V test? We've had lots of problem w/ target features in LTO w/ RV, and the module level inline asm in particular. To date we've done a few things to try and mitigate that by plumbing them through the backend. I'd like to know how this patch ends up interacting w/ that, and whether this solve some of the remaining issues, or if we need further triage for that backend.
> 
> #50591, #65090, #69780 are all related to some extent.

Yes, I'm going to add a test from #67698 for RISC-V, and check other issues that you mentioned. Not all of them are minimal, so I may need some help there.

https://github.com/llvm/llvm-project/pull/174995