[gnu-objc] Make selector order deterministic.
This probably fixes PR35277, though there may be other sources of
nondeterminism (this was the only case of iterating over a DenseMap).
It's difficult to provide a test case for this, because it shows up only
on systems with ASLR enabled.
Reviewed By: rjmccall
Subscribers: bmwiedemann, mgrang, cfe-commits
Differential Revision: https://reviews.llvm.org/D50559