contrib/llvm-project: fix clang crash compiling modules

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
MFC after: 3 days
Obtained from: https://github.com/llvm/llvm-project/pull/102855
Reviewed by: kevans, dim
Approved by: kevans (mentor)
Differential Revision: https://reviews.freebsd.org/D51041

(cherry picked from commit 55dfaeae8e9aa95f0b724d90ad2423ca1b623142)