D31187: Fix removal of out-of-line definitions.

Consider:

struct MyClass {
  void f() {}
}
MyClass::f(){} // expected error redefinition of f. #1

Some clients (eg. cling) need to call removeDecl for the redefined (#1) decl.

This patch enables us to remove the lookup entry is registered in the semantic
decl context and not in the primary decl context of the lexical decl context
where we currently are trying to remove it from.

It is not trivial to test this piece and writing a full-blown unit test seems
too much.

llvm-svn: 305799