[frontend] Initialize the diagnostic client before loading an ast file.

Issue reported by Tom Honermann!
http://llvm.org/bugs/show_bug.cgi?id=15377

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177336 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed