[clang] Fix false negative when value initializing a field annotated with [[clang::require_field_initialization]] (PR #124329)

[via cfe-commits]

higher-performance wrote:

@AaronBallman Could we get this into the 20.x release before branching? (Sorry for the last-minute PR, I just noticed the bug.)

https://github.com/llvm/llvm-project/pull/124329