Introduce a new class, UnqualifiedId, that provides a parsed

representation of a C++ unqualified-id, along with a single parsing
function (Parser::ParseUnqualifiedId) that will parse all of the
various forms of unqualified-id in C++.

Replace the representation of the declarator name in Declarator with
the new UnqualifiedId class, simplifying declarator-id parsing
considerably and providing more source-location information to
Sema. In the future, I hope to migrate all of the other
unqualified-id-parsing code over to this single representation, then
begin to merge actions that are currently only different because we
didn't have a unqualified notion of the name in the parser.

llvm-svn: 85851