Teach DeclContext how to find the primary declaration for any TagDecl

Teach DeclContext how to find the primary declaration for any TagDecl
even when we are still defining the TagDecl. This is required so that
qualified name lookup of a class name within its definition works (see
the new bits in test/SemaCXX/qualified-id-lookup.cpp).

As part of this, move the nested redefinition checking code into
ActOnTag. This gives us diagnostics earlier (when we try to perform
the nested redefinition, rather than when we try to complete the 2nd
definition) and removes some code duplication.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@62386 91177308-0d34-0410-b5e6-96231b3b80d8