Revert "Enable MSRecordLayout in the presence of external sources."

This reverts commit r201810.

It was failing these tests on my workstation:
    Clang :: CodeGen/override-layout.c
    Clang :: CodeGenCXX/override-layout.cpp
    Clang :: PCH/check-deserializations.cpp

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201823 91177308-0d34-0410-b5e6-96231b3b80d8