[nfc] speed-up enabled-Features lookups

With the growing popularity of using Features, I think it's important to
ensure our Features querying set is as fast as possible. In particular,
we sometimes may end up putting a feature check in a hot function, so
the `contains` overhead is important.

While I haven't done *any* measurements to verify this, I think a
`llvm::SmallSet<Feature, 2>` is slower than a FixedBitSet. For the
former, if the set grows more than the constant size, it switches over
to using a `std::set`. Setting the size to `numFeatures()` is not
particularly attractive or possible, because `SmallSet` is limited to a
size of 32 to be considered small, as it uses linear search over a
SmallVector to service a `contains` query.

Meanwhile, a FixedBitSet has a small constant factor overhead for
querying if the element is contained: two divisions by a
constant power-of-two, a bit shift, and a memory-read / compare. I think
that'll beat a SmallSet in all cases.

Author: kavon

include/swift/Basic/LangOptions.h

@@ -19,6 +19,7 @@
 #define SWIFT_BASIC_LANGOPTIONS_H
 
 #include "swift/Basic/Feature.h"
+#include "swift/Basic/FixedBitSet.h"
 #include "swift/Basic/FunctionBodySkipping.h"
 #include "swift/Basic/LLVM.h"
 #include "swift/Basic/Version.h"
@@ -28,7 +29,6 @@
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/None.h"
 #include "llvm/ADT/Optional.h"
-#include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
@@ -426,7 +426,7 @@ namespace swift {
     bool EnableNewOperatorLookup = false;
 
     /// The set of features that have been enabled.
-    llvm::SmallSet<Feature, 2> Features;
+    FixedBitSet<numFeatures(), Feature> Features;
 
     /// Temporary flag to support LLDB's transition to using \c Features.
     bool EnableBareSlashRegexLiterals = false;