driver: fix handling of hidden option -assume-single-threaded

It had no effect, because it was not passed to the frontend

Author: eeckstein

lib/Driver/ToolChains.cpp

@@ -168,6 +168,7 @@ static void addCommonFrontendArgs(const ToolChain &TC,
                        options::OPT_solver_shrink_unsolved_threshold);
   inputArgs.AddLastArg(arguments, options::OPT_O_Group);
   inputArgs.AddLastArg(arguments, options::OPT_RemoveRuntimeAsserts);
+  inputArgs.AddLastArg(arguments, options::OPT_AssumeSingleThreaded);
 
   // Pass on any build config options
   inputArgs.AddAllArgs(arguments, options::OPT_D);

test/Driver/options.swift

@@ -122,3 +122,6 @@
 // REMOVE_RUNTIME_ASSERTS: swift
 // REMOVE_RUNTIME_ASSERTS: -frontend {{.*}} -remove-runtime-asserts
 
+// RUN: %swiftc_driver -driver-print-jobs -assume-single-threaded %s | %FileCheck -check-prefix=ASSUME_SINGLE_THREADED %s
+// ASSUME_SINGLE_THREADED: swift
+// ASSUME_SINGLE_THREADED: -frontend {{.*}} -assume-single-threaded