[modules] Make sure enabled diagnostic pragmas inside the module don't affect the translation unit that
imports the module.

Getting diagnostic sections from modules properly working is a fixme.

rdar://13516663

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178151 91177308-0d34-0410-b5e6-96231b3b80d8