[clang] Adding `asuint` implementation to hlsl (PR #107292)

[Chris B via cfe-commits]

================
@@ -0,0 +1,42 @@
+//===----- hlsl_intrinsics.h - HLSL definitions for intrinsics ----------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+
+#ifndef _HLSL_HLSL_DETAILS_H_
+#define _HLSL_HLSL_DETAILS_H_
+
+namespace __details {
+#define HLSL_BUILTIN_ATTRIBUTES\
+  __attribute__((__always_inline__, __nodebug__)) static inline
+
+template<bool B, typename T>
+struct enable_if { };
+
+template<typename T>
+struct enable_if<true, T> {
+  using Type = T;
+};
+
+template<bool B, typename T=void>
+using enable_if_t = typename enable_if<B,T>::Type;
+
+template<typename U, typename T, int N>
+HLSL_BUILTIN_ATTRIBUTES vector<U, N> bit_cast(vector<T, N> V) {
+  _Static_assert(sizeof(U) == sizeof(T), "casting types must be same bit size.");
----------------
llvm-beanz wrote:

Try this. The 2021 language mode is still based on C++98, so enable_if_t is finacky. We can clean it up in a subsequent PR once I finish fixing the language modes.

```suggestion
HLSL_BUILTIN_ATTRIBUTES
typename enable_if<sizeof(U) == sizeof(T), vector<U, N> >::Type
bit_cast(vector<T, N> V) {
```

https://github.com/llvm/llvm-project/pull/107292