[AST] do not error on APFloat invalidOp in default mode

If FP exceptions are ignored, we should not error out of compilation
just because APFloat indicated an exception.
This is required as a preliminary step for D88238
which changes APFloat behavior for signaling NaN convert() to set
the opInvalidOp exception status.

Currently, there is no way to trigger this error because convert()
never sets opInvalidOp. FP binops that set opInvalidOp also create
a NaN, so the path to checkFloatingPointResult() is blocked by a
different diagnostic:

  // [expr.pre]p4:
  //   If during the evaluation of an expression, the result is not
  //   mathematically defined [...], the behavior is undefined.
  // FIXME: C++ rules require us to not conform to IEEE 754 here.
  if (LHS.isNaN()) {
    Info.CCEDiag(E, diag::note_constexpr_float_arithmetic) << LHS.isNaN();
    return Info.noteUndefinedBehavior();
  }
  return checkFloatingPointResult(Info, E, St);

Differential Revision: https://reviews.llvm.org/D88664