Fix duplicate class template definitions problem

Fix duplicate class template definitions problem

Summary:
We fail to import a `ClassTemplateDecl` if the "To" context already
contains a definition and then a forward decl.  This is because
`localUncachedLookup` does not find the definition.  This is not a
lookup error, the parser behaves differently than assumed in the
importer code.  A `DeclContext` contains one DenseMap (`LookupPtr`)
which maps names to lists.  The list is a special list `StoredDeclsList`
which is optimized to have one element.  During building the initial
AST, the parser first adds the definition to the `DeclContext`.  Then
during parsing the second declaration (the forward decl) the parser
again calls `DeclContext::addDecl` but that will not add a new element
to the `StoredDeclsList` rarther it simply overwrites the old element
with the most recent one.  This patch fixes the error by finding the
definition in the redecl chain.  Added tests for the same issue with
`CXXRecordDecl` and with `ClassTemplateSpecializationDecl`.  These tests
pass and they pass because in `VisitRecordDecl` and in
`VisitClassTemplateSpecializationDecl` we already use
`D->getDefinition()` after the lookup.

Reviewers: a.sidorin, xazax.hun, szepet

Subscribers: rnkovacs, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D46950

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@333082 91177308-0d34-0410-b5e6-96231b3b80d8