stdlib: repair windows build

When building for a pure windows environment, we build against a newer CRT which
does not provide direct access to __argc, __argv.  They are hidden behind
macros which use a function call.  Use the header (stdlib) to get access to
these rather than declaring them extern.  This also makes the swift runtime more
portable across various Windows environments.

From stdlib.h in ucrt 10.0.10586.0:
  #ifdef _CRT_DECLARE_GLOBAL_VARIABLES_DIRECTLY
    extern int __argc;
    extern char** __argv;
  #else
    #define __argc (*__p___argc())
    #define __argv (*__p___argv())
  #endif

The indirection is particularly nice on COFF where all module external variables
are indirectly addressed.

Author: compnerd

stdlib/public/stubs/CommandLine.cpp

@@ -90,8 +90,7 @@ extern "C" char ** _swift_stdlib_getUnsafeArgvArgc(int *outArgLen) {
   return outBuf;
 }
 #elif defined (_MSC_VER)
-extern int *__argc;
-extern char **__argv;
+#include <stdlib.h>
 
 SWIFT_CC(swift) SWIFT_RUNTIME_STDLIB_INTERFACE
 extern "C" char ** _swift_stdlib_getUnsafeArgvArgc(int *outArgLen) {