[ASTImporter] Fix handling of overriden methods during ASTImport

Summary:
When importing classes we may add a CXXMethodDecl more than once to a CXXRecordDecl when handling overrides. This patch will fix the cases we currently know about and handle the case where we are only dealing with declarations.

Differential Revision: https://reviews.llvm.org/D56936

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@352436 91177308-0d34-0410-b5e6-96231b3b80d8