Enable MSRecordLayout in the presence of external sources.

External sources shouldn't prevent the layout engine from using 
MSLayout.  If lldb were to support debugging in microsoft mode, some 
code will need to be added to MSRecordLayoutBuilder to handel external 
layouts.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201810 91177308-0d34-0410-b5e6-96231b3b80d8