Handle injected class names in the ASTImporter.

Every class as parsed by Clang has a forward declaration of itself as a member:

class A {
  class A;
  ...
}

but when the parser generates this it ensures that the RecordTypes for the two 
are the same.  This makes (among other things) inheritance work.  This patch
fixes a bug where the ASTImporter generated two separate RecordTypes when
importing the class and the contained forward declaration, and adds a test case.

Thanks to Doug Gregor for advice on this.

llvm-svn: 269551