[6.0] Fix an infinite recursion in NameMatcher if a position to resolve is in the leading whitespace before a token

[ahoppen]

• Explanation: If the name matcher was requested at a location inside whitespace in front of a token, it would try to parse the source file from there in order to match eg. a compound decl name inside the comment. However, if the location was inside leading whitespace, it would just find the token itself again and infinitely recurse.
  Limit the re-parsing to only the trivia text, which fixes the infinite recursion
• Scope: Name matcher
• Risk: Low
• Testing: Added test case
• Issue: rdar://129922267
• Reviewer: @bnbarham on Fix an infinite recursion in NameMatcher if a position to resolve is in the leading whitespace before a token #2708

[ahoppen]

@swift-ci Please test