[clang] [Clang] Add `__builtin_[ordered_]reduce_fadd` for ordered/unordered fp reductions (PR #176160)

[Benjamin Maxwell via cfe-commits]

================
@@ -3628,6 +3630,44 @@ Sema::CheckBuiltinFunctionCall(FunctionDecl *FDecl, unsigned BuiltinID,
     break;
   }
 
+  case Builtin::BI__builtin_reduce_addf:
+  case Builtin::BI__builtin_ordered_reduce_addf: {
+    if (checkArgCountRange(TheCall, 1, 2))
+      return ExprError();
----------------
MacDue wrote:

Since I've added separate builtins, I've allowed both to optionally specify the start value (with the default being -0.0f -- the FP additive identity). 

https://github.com/llvm/llvm-project/pull/176160