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.

llvm-svn: 223900