[FPEnv] Remove inaccurate comments regarding signaling NaN for isless

[eopXD]

Summary

By draft of C23 (https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2912.pdf),
the description for isless macro under 7.12.17.3 says,

The isless macro determines whether its first argument is less than its second
argument. The value of isless(x,y) is always equal to (x)< (y); however, unlike
(x) < (y), isless(x,y) does not raise the invalid floating-point exception when
x and y are unordered and neither is a signaling NaN.

isless should trap when encountering signaling NaN.

[jcranmer-intel]

This looks fine to me, but as clang is not normally my wheelhouse, I'd still prefer to see someone else approve this as well.

[efriedma]

LGTM

clang/lib/CodeGen/CGBuiltin.cpp

RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
   case Builtin::BI__builtin_isgreaterequal:
   case Builtin::BI__builtin_isless:
   case Builtin::BI__builtin_islessequal:
   case Builtin::BI__builtin_islessgreater:
   case Builtin::BI__builtin_isunordered: {
     // Ordered comparisons: we know the arguments to these are matching scalar
     // floating point values.
     CodeGenFunction::CGFPOptionsRAII FPOptsRAII(*this, E);
-    // FIXME: for strictfp/IEEE-754 we need to not trap on SNaN here.
     Value *LHS = EmitScalarExpr(E->getArg(0));
     Value *RHS = EmitScalarExpr(E->getArg(1));
 
     switch (BuiltinID) {
     default: llvm_unreachable("Unknown ordered comparison");
     case Builtin::BI__builtin_isgreater:
       LHS = Builder.CreateFCmpOGT(LHS, RHS, "cmp");
       break;