[clang] Improve ctor initializer completions.

Summary:
Instead of providing generic "args" for member and base class
initializers, tries to fetch relevant constructors and show their signatures.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: ZaMaZaN4iK, eraman, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D53654

llvm-svn: 345844