Driver: support -nostartfiles in the C++ driver

This is required to bootstrap the `-static-stdlib` support for Windows.
With this, we are able to properly build the Swift SDK both dynamically
and statically, which is needed to enable us to make further progress
towards an early swift-driver.

Author: compnerd

include/swift/Option/Options.td

@@ -2116,7 +2116,7 @@ def digester_mode:
 def nostartfiles:
   Flag<["-"], "nostartfiles">,
   Flags<[FrontendOption, DoesNotAffectIncrementalBuild, NoInteractiveOption,
-         HelpHidden, NewDriverOnlyOption]>,
+         HelpHidden]>,
   HelpText<"Do not link in the Swift language startup routines">;
 
 def gcc_toolchain: Separate<["-"], "gcc-toolchain">,

lib/Driver/UnixToolChains.cpp

@@ -266,11 +266,13 @@ toolchains::GenericUnix::constructInvocation(const DynamicLinkJobAction &job,
   getResourceDirPath(SharedResourceDirPath, context.Args,
                      /*Shared=*/!(staticExecutable || staticStdlib));
 
-  SmallString<128> swiftrtPath = SharedResourceDirPath;
-  llvm::sys::path::append(swiftrtPath,
-                          swift::getMajorArchitectureName(getTriple()));
-  llvm::sys::path::append(swiftrtPath, "swiftrt.o");
-  Arguments.push_back(context.Args.MakeArgString(swiftrtPath));
+  if (!context.Args.hasArg(options::OPT_nostartfiles)) {
+    SmallString<128> swiftrtPath = SharedResourceDirPath;
+    llvm::sys::path::append(swiftrtPath,
+                            swift::getMajorArchitectureName(getTriple()));
+    llvm::sys::path::append(swiftrtPath, "swiftrt.o");
+    Arguments.push_back(context.Args.MakeArgString(swiftrtPath));
+  }
 
   addPrimaryInputsOfType(Arguments, context.Inputs, context.Args,
                          file_types::TY_Object);

lib/Driver/WebAssemblyToolChains.cpp

@@ -116,11 +116,13 @@ toolchains::WebAssembly::constructInvocation(const DynamicLinkJobAction &job,
   SmallString<128> SharedResourceDirPath;
   getResourceDirPath(SharedResourceDirPath, context.Args, /*Shared=*/false);
 
-  SmallString<128> swiftrtPath = SharedResourceDirPath;
-  llvm::sys::path::append(swiftrtPath,
-                          swift::getMajorArchitectureName(getTriple()));
-  llvm::sys::path::append(swiftrtPath, "swiftrt.o");
-  Arguments.push_back(context.Args.MakeArgString(swiftrtPath));
+  if (!context.Args.hasArg(options::OPT_nostartfiles)) {
+    SmallString<128> swiftrtPath = SharedResourceDirPath;
+    llvm::sys::path::append(swiftrtPath,
+                            swift::getMajorArchitectureName(getTriple()));
+    llvm::sys::path::append(swiftrtPath, "swiftrt.o");
+    Arguments.push_back(context.Args.MakeArgString(swiftrtPath));
+  }
 
   addPrimaryInputsOfType(Arguments, context.Inputs, context.Args,
                          file_types::TY_Object);

lib/Driver/WindowsToolChains.cpp

@@ -144,14 +144,16 @@ toolchains::Windows::constructInvocation(const DynamicLinkJobAction &job,
                                                    getTriple().getArchName()));
   }
 
-  SmallString<128> SharedResourceDirPath;
-  getResourceDirPath(SharedResourceDirPath, context.Args, /*Shared=*/true);
-
-  SmallString<128> swiftrtPath = SharedResourceDirPath;
-  llvm::sys::path::append(swiftrtPath,
-                          swift::getMajorArchitectureName(getTriple()));
-  llvm::sys::path::append(swiftrtPath, "swiftrt.obj");
-  Arguments.push_back(context.Args.MakeArgString(swiftrtPath));
+  if (!context.Args.hasArg(options::OPT_nostartfiles)) {
+    SmallString<128> SharedResourceDirPath;
+    getResourceDirPath(SharedResourceDirPath, context.Args, /*Shared=*/true);
+
+    SmallString<128> swiftrtPath = SharedResourceDirPath;
+    llvm::sys::path::append(swiftrtPath,
+                            swift::getMajorArchitectureName(getTriple()));
+    llvm::sys::path::append(swiftrtPath, "swiftrt.obj");
+    Arguments.push_back(context.Args.MakeArgString(swiftrtPath));
+  }
 
   addPrimaryInputsOfType(Arguments, context.Inputs, context.Args,
                          file_types::TY_Object);