[DiagnosticVerifier] skip over unrelated diags in expansions

[hnrklssn]

This adds support for the following case:

expected-expansion@:1:2{{
  expected-warning@3{{foo}}
  expected-sometimes-warning@4{{bar}}
  expected-warning@5{{foobar}}
}}

This would work when parsing the additional "sometimes-" prefix, but for invokations that didn't parse that prefix the "}}" on that line would close the expansion, and expected-warning@5{{foobar}} would be parsed as a top level statement.

[hnrklssn]

@swift-ci please smoke test