[CodeComplete] Member completion for concept-constrained types.

Summary:
The basic idea is to walk through the concept definition, looking for
t.foo() where t has the constrained type.

In this patch:
 - nested types are recognized and offered after ::
 - variable/function members are recognized and offered after the correct
   dot/arrow/colon trigger
 - member functions are recognized (anything directly called). parameter
   types are presumed to be the argument types. parameters are unnamed.
 - result types are available when a requirement has a type constraint.
   These are printed as constraints, except same_as<T> which prints as T.

Not in this patch:
 - support for merging/overloading when two locations describe the same member.
   The last one wins, for any given name. This is probably important...
 - support for nested template members (T::x<int>)
 - support for completing members of (instantiations of) template template parameters

Reviewers: nridge, saar.raz

Subscribers: mgrang, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73649