Fixed the AST importer ot use redeclaration contexts properly.

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223912 91177308-0d34-0410-b5e6-96231b3b80d8