[CodeCompletion] Provide member completions for dependent expressions whose

type is a TemplateSpecializationType or InjectedClassNameType

Fixes PR30847. Partially fixes PR20973 (first position only).

PR17614 is still not working, its expression has the dependent
builtin type. We'll have to teach the completion engine how to "resolve"
dependent expressions to fix it.

rdar://29818301

llvm-svn: 302797