Add missing const_cast in UTF16Stream.

Summary:
The LLVM `ConvertUTF8ToUTF16` function takes a `llvm::UTF16 *` without
a `const` qualifier as the "end" argument. It is never written to.

Silence a warning in the build by adding an explicit `const_cast` to
show what's already happening implicitly.

Reviewed By: tmikov

Differential Revision: D19825240

fbshipit-source-id: 3ec4f2188c67e33977405a586cfff4698beb55b4

lib/Support/UTF16Stream.cpp

@@ -48,7 +48,7 @@ bool UTF16Stream::refill() {
       &utf8Begin_,
       utf8End_,
       (llvm::UTF16 **)&out,
-      (llvm::UTF16 *)end_,
+      (llvm::UTF16 *)const_cast<char16_t *>(end_),
       llvm::lenientConversion);
 
   if (cRes != llvm::ConversionResult::targetExhausted) {