[gnu-objc] Make selector order deterministic.

Summary:
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.

Reviewers: rjmccall

Reviewed By: rjmccall

Subscribers: bmwiedemann, mgrang, cfe-commits

Differential Revision: https://reviews.llvm.org/D50559

llvm-svn: 339668