[verify] Add support for location markers in directives.

A marker (matching /#[A-Za-z0-9_-]/) is specified by attaching a comment
containing the marker to the line at which the diagnostic is expected,
and then can be referenced from an expected-* directive after an @:

  foo // #1
  // expected-error@#1 {{undeclared identifier 'foo'}}

The intent is for markers to be used in situations where relative line
numbers are currently used, to avoid the need to renumber when the test
case is rearranged.

llvm-svn: 358326