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/OSLogOptimization.cpp

@@ -196,7 +196,7 @@ public:
   SILInstruction *beginInstruction;
 
   /// Instructions that mark the end points of constant evaluation.
-  SmallSetVector<SILInstruction *, 2> endInstructions;
+  llvm::SmallSetVector<SILInstruction *, 2> endInstructions;
 
 private:
   /// SIL values that were found to be constants during