Fixed the AST importer ot use redeclaration contexts properly.

To ensure that we don't import duplicates, the ASTImporter often
checks the DeclContext for a Decl before importing it to see if
a Decl with the same name is already present.  

This is problematic if the Decl is inside a transparent context
like an extern "C" block.  Lookup isn't allowed on such contexts,
and in fact they assert() if you do that.  So instead we look at
the redeclaration context -- the containing context that can be
used safely for these kinds of checks -- instead.

llvm-svn: 223912