[ASTImporter] Improve handling of incomplete types

ASTImporter has some bugs when it's importing types 
that themselves come from an ExternalASTSource. This 
is exposed particularly in the behavior when 
comparing complete TagDecls with forward 
declarations. This patch does several things:

- Adds a test case making sure that conflicting 
  forward-declarations are resolved correctly;
- Extends the clang-import-test harness to test 
  two-level importing, so that we make sure we 
  complete types when necessary; and
- Fixes a few bugs I found this way. Failure to 
  complete types was one; however, I also discovered 
  that complete RecordDecls aren't properly added to 
  the redecls chain for existing forward 
  declarations.

llvm-svn: 302975