[Sema][ObjC] Use SmallSetVector to fix a failing test on the reverse

iteration bot.

This commit reverts r315639, which was causing clang to print
diagnostics that weren't printed before. Instead, it declares
OverrideSearch::Overridden as a SmallSetVector to fix the
non-deterministic behavior r315639 was trying to fix.

rdar://problem/36445528

llvm-svn: 324425