[CodeComplete] Include more text into typed chunks of pattern completions

Summary:
To allow filtering on any of the words in the editors.
In particular, the following completions were changed:
    - 'using namespace <#name#>'
      Typed text before: 'using', after: 'using namespace'.
    - 'else if (#<condition#>)'
      Before: 'else', after: 'else if'.
    - 'using typename <#qualifier#>::<#name#>'
      Before: 'using', after: 'using typename'.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62615

llvm-svn: 362479