Fix the rewriter that broke with r149987.

r149987 changed the way parsing happens inside an @implementation;
it aggregates the declarations inside and reports them together as a DeclGroup.
This had the side effect that function declarations were reported together with
their definition, while the rewriter expected for function declarations to be
reported immediately to the consumer and thus not have a body.

Fix this by having the rewriter actually check with isThisDeclarationADefinition()
to make sure the body comes from the current decl before rewriting it.

llvm-svn: 150325