Improve diagnostics when the parser encounters a declarator with an

unknown type name, e.g.,

  foo::bar x;

when "bar" does not refer to a type in "foo". 

With this change, the parser now calls into the action to perform
diagnostics and can try to recover by substituting in an appropriate
type. For example, this allows us to easily diagnose some missing
"typename" specifiers, which we now do:

  test/SemaCXX/unknown-type-name.cpp:29:1: error: missing 'typename'
        prior to dependent type name 'A<T>::type'
  A<T>::type A<T>::f() { return type(); }
  ^~~~~~~~~~
  typename 

Fixes PR3990.

llvm-svn: 84053