stdlib: make CommandLine inspection work

This implements commandline access on Windows by using the Windows Shell
API to access the commandline information and making it available in
Swift.  This is needed for the correct invocation of the child process
in the unit tests.

Author: compnerd

stdlib/public/stubs/CommandLine.cpp

@@ -29,6 +29,12 @@
 #include "../SwiftShims/RuntimeStubs.h"
 #include "../SwiftShims/GlobalObjects.h"
 
+#if defined(_WIN32)
+#define WIN32_LEAN_AND_MEAN
+#include <Windows.h>
+#include <shellapi.h>
+#endif
+
 // Backing storage for overrides of `Swift.CommandLine.arguments`.
 static char **_swift_stdlib_ProcessOverrideUnsafeArgv = nullptr;
 static int _swift_stdlib_ProcessOverrideUnsafeArgc = 0;
@@ -101,8 +107,53 @@ char ** _swift_stdlib_getUnsafeArgvArgc(int *outArgLen) {
     return _swift_stdlib_ProcessOverrideUnsafeArgv;
   }
 
-  *outArgLen = __argc;
-  return __argv;
+  *outArgLen = 0;
+
+
+  LPWSTR *szArgList;
+  int nArgs;
+  szArgList = CommandLineToArgvW(GetCommandLineW(), &nArgs);
+  if (szArgList == nullptr)
+    return nullptr;
+
+  std::vector<char *> argv;
+  for (int i = 0; i < nArgs; ++i) {
+    int szBufferSize = WideCharToMultiByte(CP_UTF8, WC_ERR_INVALID_CHARS,
+                                           szArgList[i], -1, nullptr, 0,
+                                           nullptr, nullptr);
+    if (szBufferSize == 0) {
+      for (char *arg : argv)
+        free(arg);
+      return nullptr;
+    }
+
+    char *buffer = static_cast<char *>(calloc(static_cast<size_t>(szBufferSize),
+                                              sizeof(char)));
+    if (buffer == nullptr) {
+      for (char *arg : argv)
+        free(arg);
+      return nullptr;
+    }
+
+    if (!WideCharToMultiByte(CP_UTF8, WC_ERR_INVALID_CHARS, szArgList[i], -1,
+                             buffer, szBufferSize, nullptr, nullptr)) {
+      for (char *arg : argv)
+        free(arg);
+      return nullptr;
+    }
+
+    argv.push_back(buffer);
+  }
+
+  LocalFree(szArgList);
+
+  char **args = static_cast<char **>(calloc(argv.size() + 1, sizeof(char *)));
+  std::copy(argv.begin(), argv.end(), args);
+  args[argv.size()] = nullptr;
+
+  assert(argv.size() < INT_MAX && "overflow");
+  *outArgLen = static_cast<int>(argv.size());
+  return args;
 }
 #elif defined(__FreeBSD__)
 #include <errno.h>