Fix loader crash when UTF8 space exceeds UTF16 space.

The code that converts the clink_???.exe UTF16 command line to UTF8 took
an unsafe shortcut and assumed the UTF8 representations would always
take fewer bytes than the UTF16 representations.

But that is of course not true, and the code could crash.

This has been fixed.

clink/app/src/loader/loader.cpp

@@ -235,12 +235,26 @@ int loader(int argc, char** argv)
 int loader_main_impl()
 {
     int argc = 0;
-    LPWSTR* argv = CommandLineToArgvW(GetCommandLineW(), &argc);
+    LPWSTR* argvw = CommandLineToArgvW(GetCommandLineW(), &argc);
+
+    char** argv = static_cast<char**>(calloc(argc + 1, sizeof(char*)));
     for (int i = 0; i < argc; ++i)
-        to_utf8((char*)argv[i], 0xffff, argv[i]);
+    {
+        const int needed = WideCharToMultiByte(CP_UTF8, 0, argvw[i], -1, nullptr, 0, nullptr, nullptr);
+        if (!needed)
+            return 1;
+        argv[i] = static_cast<char*>(malloc(needed));
+        WideCharToMultiByte(CP_UTF8, 0, argvw[i], -1, argv[i], needed, nullptr, nullptr);
+    }
+    assert(argv[argc] == nullptr);
 
-    int ret = loader(argc, (char**)argv);
+    LocalFree(argvw);
+
+    int ret = loader(argc, argv);
+
+    for (int i = 0; i < argc; ++i)
+        free(argv[i]);
+    free(argv);
 
-    LocalFree(argv);
     return ret;
 }