Make the use of libSwiftScan opt-in, defaulting to swift-frontend.

This library is unavailable in some contexts and on some architectures. For now, let's have the swift-frontend flow be the default.

Author: artemcm

Sources/SwiftDriver/Jobs/EmitSupportedFeaturesJob.swift

@@ -57,15 +57,17 @@ extension Driver {
                                                fileSystem: FileSystem,
                                                executor: DriverExecutor, env: [String: String])
   throws -> Set<String> {
-    // If libSwiftScan library is present, use it to query
-    let swiftScanLibPath = try Self.getScanLibPath(of: toolchain,
-                                                   hostTriple: hostTriple,
-                                                   env: env)
+    if env["SWIFT_DRIVER_USE_SWIFT_SCAN_FEATURES_QUERY"] != nil {
+      // If libSwiftScan library is present, use it to query
+      let swiftScanLibPath = try Self.getScanLibPath(of: toolchain,
+                                                     hostTriple: hostTriple,
+                                                     env: env)
 
-    if fileSystem.exists(swiftScanLibPath) {
-      let libSwiftScanInstance = try SwiftScan(dylib: swiftScanLibPath)
-      if libSwiftScanInstance.canQuerySupportedArguments() {
-        return try libSwiftScanInstance.querySupportedArguments()
+      if fileSystem.exists(swiftScanLibPath) {
+        let libSwiftScanInstance = try SwiftScan(dylib: swiftScanLibPath)
+        if libSwiftScanInstance.canQuerySupportedArguments() {
+          return try libSwiftScanInstance.querySupportedArguments()
+        }
       }
     }