Only perform checking of the predefines buffer when loading a

Only perform checking of the predefines buffer when loading a
precompiled header. Previously, we were trying to gather predefines
buffers from all kinds of AST files (which doesn't make sense) and
were performing some validation when AST files were loaded as main
files.

With these tweaks, using PCH files that import modules no longer fails
immediately (due to mismatched predefines buffers). However, module
visibility is lost, so this feature does not yet work.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@145709 91177308-0d34-0410-b5e6-96231b3b80d8