[Frontend] Avoid running plugins during code completion parse

[Frontend] Avoid running plugins during code completion parse

The parsing that is done for code completion is a special case that will
discard any generated diagnostics, so avoid running plugins for this
case in the first place to avoid performance penalties due to the
plugins.

A scenario for this is for example libclang with extra plugins like tidy.

Patch by Nikolai Kosjar

Differential Revision: https://reviews.llvm.org/D46050

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@332469 91177308-0d34-0410-b5e6-96231b3b80d8