This patch continues parser-level implementation of designators:

1. It introduces new parser level abstractions for designators
   that are used to communicate between parser and sema.
2. This fixes a FIXME where "identifier ':'" was considered to be
   a designator even if it wasn't the first in a designator list.
3. In the "identifier ':'" case, it actually builds the designator
   representation.

llvm-svn: 58205