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/AST/Module.cpp

@@ -626,7 +626,7 @@ void SourceLookupCache::lookupVisibleDecls(ImportPath::Access AccessPath,
   // 'names' in macro role attributes). Since expansions are cached, it doesn't
   // cause duplicated expansions, but different 'unexpandedDecl' may report the
   // same 'ValueDecl'.
-  SmallSetVector<ValueDecl *, 4> macroExpandedDecls;
+  llvm::SmallSetVector<ValueDecl *, 4> macroExpandedDecls;
   for (MissingDecl *unexpandedDecl : unexpandedDecls) {
     unexpandedDecl->forEachMacroExpandedDecl([&](ValueDecl *vd) {
       macroExpandedDecls.insert(vd);