Add SmallSetVector replacement

llvm::SmallSetVector changed semantics
(https://reviews.llvm.org/D152497) resulting in build failures in Swift.
The old semantics allowed usage of types that did not have an
`operator==` because `SmallDenseSet` uses `DenseSetInfo<T>::isEqual` to
determine equality. The new implementation switched to using
`std::find`, which internally uses `operator==`. This type is used
pretty frequently with `swift::Type`, which intentionally deletes
`operator==` as it is not the canonical type and therefore cannot be
compared in normal circumstances.

This patch adds a new type-alias to the Swift namespace that provides
the old semantic behavior for `SmallSetVector`. I've also gone through
and replaced usages of `llvm::SmallSetVector` with the
`Swift::SmallSetVector` in places where we're storing a type that
doesn't implement or explicitly deletes `operator==`. The changes to
`llvm::SmallSetVector` should improve compile-time performance, so I
left the `llvm::SmallSetVector` where possible.

lib/SILOptimizer/Mandatory/MoveOnlyObjectCheckerUtils.cpp

@@ -69,7 +69,7 @@ using namespace swift::siloptimizer;
 
 bool swift::siloptimizer::searchForCandidateObjectMarkMustChecks(
     SILFunction *fn,
-    SmallSetVector<MarkMustCheckInst *, 32> &moveIntroducersToProcess,
+    llvm::SmallSetVector<MarkMustCheckInst *, 32> &moveIntroducersToProcess,
     DiagnosticEmitter &emitter) {
   bool localChanged = false;
   for (auto &block : *fn) {