contrib/llvm-project: fix clang crash compiling modules

[ivy]

Summary

clang++ may crash when compiling certain C++20 modules. Backport the
fix from LLVM upstream.

This fixes LLVM bug 102684:
https://github.com/llvm/llvm-project/issues/102684.

PR: 287803
Obtained from: https://github.com/llvm/llvm-project/pull/102855

[dim]

Can you put a MFC after: 3 days header in the commit message, please? (I tend to use 3 days for upstream commits that are trivial; this one seems a bit more extensive so maybe you can extend the MFC period to one or two weeks)

[ivy]

In D51041#1164931, @dim wrote:

Can you put a MFC after: 3 days header in the commit message, please?

i've added 3 days to my local commit message. i've tested this with buildworld and with the commit that produced the problem, and it's in a fairly niche feature (C++20 modules) so i'm happy with 3 days personally, but i could do longer if you think it's appropriate.

[dim]

3 days is fine, if it works. :)