[CodeComplete] Filter override completions by function name

Summary:
We put only part of the signature starting with a function name into "typed text"
chunks now, previously the whole signature was "typed text".

This leads to meaningful fuzzy match scores, giving better signals to
compare with other completion items.

Ideally, we would not display the result type to the user, but that requires adding
a new kind of completion chunk.

Reviewers: kadircet

Reviewed By: kadircet

Subscribers: jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62298

llvm-svn: 361623