[clang] [Clang] Wide delimiters ('{{{') for expect strings (PR #77326)

[Erich Keane via cfe-commits]

erichkeane wrote:

Can you update https://clang.llvm.org/docs/InternalsManual.html#verifying-diagnostics as well please?  It should be in-repo, but we should make sure that keeps up to date here.  Else, LGTM, thanks!

https://github.com/llvm/llvm-project/pull/77326