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

Prior to this commit, it was impossible to use the simple string
matching directives to look for any content that contains unbalanced
`{{` `}}` pairs, such as:

```
// expected-note {{my_struct{{1}, 2}}}
```

Which would parse like so:

```
             "nested" brace v
// expected-note {{my_struct{{1}, 2}}}
          closes the nested brace  ^ |
                            trailing }
```

And the frontend would complain 'cannot find end ('}}') of expected'.

At this snapshot, VerifyDiagnosticConsumer's parser now counts the
opening braces and looks for a matching length of closing sigils,
allowing the above to be written as:

```
// expected-note {{{my_struct{{1}, 2}}}}
   opening brace |-|                 |-|
  closing brace is '}}}', found here ^
```

This came about as a result of this discussion:
https://github.com/llvm/llvm-project/pull/74852#discussion_r1443117644

cc @erichkeane