[CodeComplete] Fix ASTUnit cached completion of macros from preamble, broken in r342528

Summary:
The problem is the default LoadExternal with no completer, which happens when
loading global results.

Reviewers: ilya-biryukov, nik

Subscribers: arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64864

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@366409 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed