Nocompile fixes for new clang only

Clang changed some of the diagnostics it outputs, so we need up update
our nocompile tests to match. To be compatible with pinned clang as well
as ToT, some of the new errors need to be optional (expecting 0-1
instances); we can remove this during the next clang roll.

Bug: 430264113
Change-Id: I70f1b1344ab0714858dfe53594c050973f2f3510
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6715086
Commit-Queue: Devon Loehr <dloehr@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1483909}