[libc] [llvm] [libc] Remove obsolete LIBC_HAS_BUILTIN macro (PR #86554)

[Guillaume Chatelet via llvm-commits]

================
@@ -71,10 +71,11 @@ template <typename T> struct Atomic {
 
   T load(MemoryOrder mem_ord = MemoryOrder::SEQ_CST,
          [[maybe_unused]] MemoryScope mem_scope = MemoryScope::DEVICE) {
-    if constexpr (LIBC_HAS_BUILTIN(__scoped_atomic_load_n))
-      return __scoped_atomic_load_n(&val, int(mem_ord), (int)(mem_scope));
-    else
-      return __atomic_load_n(&val, int(mem_ord));
+#if __has_builtin(__scoped_atomic_load_n)
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gchatelet wrote:

Why convert `if constexpr` into preprocessor directive?

Can we revert this to
```
if constexpr (__has_builtin(__scoped_atomic_load_n))
  return __scoped_atomic_load_n(&val, int(mem_ord), (int)(mem_scope));
else
  return __atomic_load_n(&val, int(mem_ord));
```

here and elsewhere?

https://github.com/llvm/llvm-project/pull/86554