[CodeComplete] Clearly distinguish signature help and code completion.

Summary:
Code completion in clang is actually a mix of two features:
- Code completion is a familiar feature. Results are exposed via the
  CodeCompleteConsumer::ProcessCodeCompleteResults callback.
- Signature help figures out if the current expression is an argument of
  some function call and shows corresponding signatures if so.
  Results are exposed via CodeCompleteConsumer::ProcessOverloadCandidates.

This patch refactors the implementation to untangle those two from each
other and makes some naming tweaks to avoid confusion when reading the
code.

The refactoring is required for signature help fixes, see D51038.

The only intended behavior change is the order of callbacks.
ProcessOverloadCandidates is now called before ProcessCodeCompleteResults.

Reviewers: sammccall, kadircet

Reviewed By: sammccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D51782

llvm-svn: 341660