Put statement expression decls in the enclosing code DeclContext

We already check that statement expressions are in a function or block,
but we didn't do anything with that information. Now we use that
DeclContext for the duration of the statement expression. Otherwise,
we'd treat statement expression locals as static data members and go
into the weeds.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@235335 91177308-0d34-0410-b5e6-96231b3b80d8