[clang] [Clang] Add attribute for consteval builtins; Declare constexpr builtins as constexpr in C++ (PR #91894)

[Erich Keane via cfe-commits]

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@@ -14,13 +14,18 @@ void __builtin_va_copy(double d);
 // expected-error at +2 {{cannot redeclare builtin function '__builtin_va_end'}}
 // expected-note at +1 {{'__builtin_va_end' is a builtin with type}}
 void __builtin_va_end(__builtin_va_list);
-// RUN: %clang_cc1 %s -fsyntax-only -verify 
-// RUN: %clang_cc1 %s -fsyntax-only -verify -x c
 
 void __va_start(__builtin_va_list*, ...);
 
+          void *__builtin_assume_aligned(const void *, size_t, ...);
 #ifdef __cplusplus
-void *__builtin_assume_aligned(const void *, size_t, ...) noexcept;
-#else
-void *__builtin_assume_aligned(const void *, size_t, ...);
+constexpr void *__builtin_assume_aligned(const void *, size_t, ...);
+          void *__builtin_assume_aligned(const void *, size_t, ...) noexcept;
+constexpr void *__builtin_assume_aligned(const void *, size_t, ...) noexcept;
+          void *__builtin_assume_aligned(const void *, size_t, ...) throw();
+constexpr void *__builtin_assume_aligned(const void *, size_t, ...) throw();
+
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erichkeane wrote:

>From reading through that (and trying to search my memory): A number of our `LIBBUILTIN` builtins are just replacements for things that would otherwise be implemented in the  library.  

So we need to allow them to be redeclared because they might be implemented in the library.

However, I would be unopposed to a patch that makes declaring a `__builtin` spelled as  reserved identifier ill-formed.

https://github.com/llvm/llvm-project/pull/91894