Handle --driver-mode= when using new driver

Previously the driver mode was always set based on the executable name,
ignoring any manually passed flag, now this flag is preferred if it
exists.

Fixes swiftlang#60600

Author: keith

lib/DriverTool/driver.cpp

@@ -193,6 +193,7 @@ static int run_driver(StringRef ExecName,
   bool isRepl = false;
 
   // Handle integrated tools.
+  StringRef DriverModeArg;
   if (argv.size() > 1) {
     StringRef FirstArg(argv[1]);
 
@@ -219,6 +220,8 @@ static int run_driver(StringRef ExecName,
     if (FirstArg == "repl") {
       isRepl = true;
       argv = argv.drop_front();
+    } else if (FirstArg.startswith("--driver-mode=")) {
+      DriverModeArg = FirstArg;
     }
   }
 
@@ -239,7 +242,9 @@ static int run_driver(StringRef ExecName,
       std::vector<const char *> subCommandArgs;
       // Rewrite the program argument.
       subCommandArgs.push_back(NewDriverPath.c_str());
-      if (ExecName == "swiftc") {
+      if (!DriverModeArg.empty()) {
+        subCommandArgs.push_back(DriverModeArg.data());
+      } else if (ExecName == "swiftc") {
         subCommandArgs.push_back("--driver-mode=swiftc");
       } else {
         assert(ExecName == "swift");