DebugNamesDWARFIndex: Implement DWARFDeclContext variant of GetTypes method

This method is used to find complete definitions of a type when one
parses a compile unit with only forward declaration available.

Since it is only accessed from DWARFASTParserClang, it was not
possible/easy to trigger this codepath from lldb-test. Therefore, I
adapt add a debug-names variant to an existing dotest test to cover this
scenario.

llvm-svn: 334516