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/Sema/ConstraintSystem.cpp

@@ -4537,7 +4537,7 @@ static bool diagnoseConflictingGenericArguments(ConstraintSystem &cs,
     auto *typeVar = entry.first;
     auto GP = entry.second;
 
-    llvm::SmallSetVector<Type, 4> arguments;
+    swift::SmallSetVector<Type, 4> arguments;
     for (const auto &solution : solutions) {
       auto type = solution.typeBindings.lookup(typeVar);
       // Type variables gathered from a solution's type binding context may not
@@ -5083,7 +5083,7 @@ static bool diagnoseContextualFunctionCallGenericAmbiguity(
   // So let's try to collect the set of fixed types for the generic parameter
   // from all the closure contextual fix/solutions and if there are more than
   // one fixed type diagnose it.
-  llvm::SmallSetVector<Type, 4> genericParamInferredTypes;
+  swift::SmallSetVector<Type, 4> genericParamInferredTypes;
   for (auto &fix : contextualFixes)
     genericParamInferredTypes.insert(fix.first->getFixedType(resultTypeVar));