[CodeComplete] Properly determine qualifiers of 'this' in a lambda

Summary:
The clang used to pick up the qualifiers of the lamba's call operator
(which is always const) and fail to show non-const methods of 'this' in
completion results.

Reviewers: kadircet

Reviewed By: kadircet

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D55885

llvm-svn: 349655