Only define the various Macros features when the compiler supports them

We were enabling the `$Macros` feature unconditionally, even when the
compiler itself doesn't support macros (because it's missing
swift-syntax). Change this to only enable the feature when the
compiler supports it.

Author: DougGregor

include/swift/Basic/Features.def

@@ -94,11 +94,11 @@ SUPPRESSIBLE_LANGUAGE_FEATURE(PrimaryAssociatedTypes2, 346, "Primary associated
 SUPPRESSIBLE_LANGUAGE_FEATURE(UnavailableFromAsync, 0, "@_unavailableFromAsync", true)
 SUPPRESSIBLE_LANGUAGE_FEATURE(NoAsyncAvailability, 340, "@available(*, noasync)", true)
 LANGUAGE_FEATURE(BuiltinIntLiteralAccessors, 368, "Builtin.IntLiteral accessors", true)
-LANGUAGE_FEATURE(Macros, 0, "Macros", true)
+LANGUAGE_FEATURE(Macros, 0, "Macros", hasSwiftSwiftParser)
 SUPPRESSIBLE_LANGUAGE_FEATURE(
     FreestandingExpressionMacros, 382, "Expression macros",
-    true)
-LANGUAGE_FEATURE(AttachedMacros, 389, "Attached macros", true)
+        hasSwiftSwiftParser)
+LANGUAGE_FEATURE(AttachedMacros, 389, "Attached macros", hasSwiftSwiftParser)
 LANGUAGE_FEATURE(MoveOnly, 390, "noncopyable types", true)
 
 UPCOMING_FEATURE(ConciseMagicFile, 274, 6)

lib/Parse/ParseIfConfig.cpp

@@ -555,6 +555,11 @@ class EvaluateIfConfigCondition :
 
     // Check whether this is any one of the known compiler features.
     const auto &langOpts = Ctx.LangOpts;
+#if SWIFT_SWIFT_PARSER
+    const bool hasSwiftSwiftParser = true;
+#else
+    const bool hasSwiftSwiftParser = false;
+#endif
     bool isKnownFeature = llvm::StringSwitch<bool>(Name)
 #define LANGUAGE_FEATURE(FeatureName, SENumber, Description, Option) \
         .Case("$" #FeatureName, Option)