stdlib: change the console to UTF-8 on start

This adjusts the Windows console to switch the codepage to UTF-8.  This
is important as the default codepage (CP437) does not allow for UTF-8
output, but expects ASCII.  However, strings in Swift are assumed to be
UTF-8, which means that there is now a conversion mismatch.

Because the console mode persists beyond the duration of the application
as it is state local to the console and not the C runtime, we should
restore the state of the console before termination.  We do this by
registering a termination handler via `atexit`.  This means that an
abnormal termination (e.g. via `fatalError`) will irrevocably alter the
state of the console (interestingly enough, `chcp` will still report the
original console codepage even though the console will internally be set
to UTF-8).

Fixes: SR-13807

Author: compnerd

stdlib/public/stubs/LibcShims.cpp

@@ -31,11 +31,20 @@
 
 #include "../SwiftShims/LibcShims.h"
 
+#if defined(_WIN32)
+static void __attribute__((__constructor__))
+_swift_stdlib_configure_console_mode(void) {
+  static UINT uiPrevConsoleCP = GetConsoleOutputCP();
+  atexit([]() { SetConsoleOutputCP(uiPrevConsoleCP); });
+  SetConsoleOutputCP(CP_UTF8);
+}
+#endif
+
 SWIFT_RUNTIME_STDLIB_INTERNAL
 __swift_size_t _swift_stdlib_fwrite_stdout(const void *ptr,
-                                                  __swift_size_t size,
-                                                  __swift_size_t nitems) {
-    return fwrite(ptr, size, nitems, stdout);
+                                           __swift_size_t size,
+                                           __swift_size_t nitems) {
+  return fwrite(ptr, size, nitems, stdout);
 }
 
 SWIFT_RUNTIME_STDLIB_SPI