[Frontend] SupportedFeatures: print "enabled_in" as a string

This is future-proofing the version part of the upcoming
feature. There currently no features that require that
but it's possible that they'd be added in the future.

Author: xedin

lib/Basic/SupportedFeatures.cpp

@@ -52,7 +52,7 @@ void printSupportedFeatures(llvm::raw_ostream &out) {
       out << ", \"migratable\": true";
     }
     if (auto version = feature.getLanguageVersion()) {
-      out << ", \"enabled_in\": " << *version;
+      out << ", \"enabled_in\": \"" << *version << "\"";
     }
     out << " }";
   };
@@ -71,4 +71,4 @@ void printSupportedFeatures(llvm::raw_ostream &out) {
 }
 
 } // end namespace features
-} // end namespace swift
+} // end namespace swift

test/Frontend/print-supported-features.swift

@@ -2,7 +2,7 @@
 
 // CHECK: "features": {
 // CHECK-NEXT:   "upcoming": [
-// CHECK:     { "name": "{{.*}}"{{, "migratable": true}}, "enabled_in": {{[0-9]+}} }
+// CHECK:     { "name": "{{.*}}"{{, "migratable": true}}, "enabled_in": "{{.*}}" }
 // CHECK:   ],
 // CHECK-NEXT:   "experimental": [
 // CHECK:     { "name": "{{.*}}"{{, "migratable": true}} }