Make the UTF8-to-wide functions into SPI rather than internal.

They're useful functions to have; I don't think we want them to be API, but
having them as SPI is conceivably useful for other purposes, and avoids everyone
rolling their own copy.

rdar://103397975

Author: al45tair

include/swift/Runtime/Win32.h

@@ -39,7 +39,7 @@
 ///          for freeing this string with @c free() when done with it.
 ///
 /// If @a str cannot be converted to UTF-8, @c nullptr is returned.
-SWIFT_RUNTIME_STDLIB_INTERNAL
+SWIFT_RUNTIME_STDLIB_SPI
 char *_swift_win32_copyUTF8FromWide(const wchar_t *str);
 
 /// Convert a UTF-8 string to a wide string.
@@ -50,7 +50,7 @@ char *_swift_win32_copyUTF8FromWide(const wchar_t *str);
 ///          for freeing this string with @c free() when done with it.
 ///
 /// If @a str cannot be converted to UTF-16, @c nullptr is returned.
-SWIFT_RUNTIME_STDLIB_INTERNAL
+SWIFT_RUNTIME_STDLIB_SPI
 wchar_t *_swift_win32_copyWideFromUTF8(const char *str);
 
 /// Configure the environment to allow calling into the Debug Help library.