Generate parent context id from Decl* instead of DeclContext*.

Because of multiple inheritance, a DeclContext pointer does not produce
the same pointer representation as a Decl pointer that references the
same AST Node.

When dumping the parentDeclContextId field of a node, convert the pointer
to Decl* first, so the id can be used to find the AST node it references.

Patch by Bert Belder.

llvm-svn: 370970