[CodeCompletion] Enable signature help when initializing class/struct/union members.

Summary:
Factors out member decleration gathering and uses it in parsing to call signature
help. Doesn't support signature help for base class constructors, the code was too
coupled with diagnostic handling, but still can be factored out but just needs
more afford.

Reviewers: sammccall, ilya-biryukov, ioeric

Reviewed By: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D51917

llvm-svn: 341949