Small patch to silence warnings

- Remove unreachable `fatalError`
- Fix unreachable `return` statement

Author: artemcm

Sources/SwiftDriverExecution/SwiftDriverExecutor.swift

@@ -47,7 +47,6 @@ public final class SwiftDriverExecutor: DriverExecutor {
       }
 
       try exec(path: arguments[0], args: arguments)
-      fatalError("unreachable, exec always throws on failure")
     } else {
       var childEnv = env
       childEnv.merge(job.extraEnvironment, uniquingKeysWith: { (_, new) in new })

Tests/TestUtilities/DriverExtensions.swift

@@ -53,10 +53,9 @@ private let cachedSDKPath: Result<String, Error>? = {
   }
   // Assume that if neither of the environment variables are set, we are
   // using a build-tree version of the swift frontend, and so we do not set
-  // `-sdk`. Fallthrough to return `nil`.
-  #endif
-
-  #if os(macOS)
+  // `-sdk`.
+  return nil
+  #elseif os(macOS)
   return Result {
     if let pathFromEnv = ProcessEnv.vars["SDKROOT"] {
       return pathFromEnv
@@ -79,7 +78,7 @@ private let cachedSDKPath: Result<String, Error>? = {
     }
     return path.spm_chomp()
   }
-  #endif
-
+  #else
   return nil
+  #endif
 }()