Don't show deleted function (constructor) candidates for code completion

In case of copy constructor is implicitly deleted it's still shown.
PR34402 describes a way to reproduce that.

Patch by Ivan Donchevskii!

Differential Revision: https://reviews.llvm.org/D37435

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312785 91177308-0d34-0410-b5e6-96231b3b80d8