Made the ASTImporter resilient if it can't import

Made the ASTImporter resilient if it can't import
SourceLocations.  LLDB rarely has the same files
mapped into the target AST context as the source
AST context, so the ASTImporter shouldn't expect
to see those files there.

This started to become a problem when importing
entities from modules -- these have proper source
locations, in contrast to all the ASTs LLDB
creates which have empty ones.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223900 91177308-0d34-0410-b5e6-96231b3b80d8