[ASTImporter] Improve handling of incomplete types

[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