[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

llvm-svn: 366409