[flang] Fix folding of EXPONENT() intrinsic function

[klausler]

Summary

The definition of the EXPONENT() intrinsic function differs by one
from the real arithmetic folding templates concept of an unbiased
exponent, and also needs special handling for zero. Fix, and add
more tests.

[PeteSteinfeld]

All builds, tests, and looks good.

flang/include/flang/Evaluate/real.h

   template <typename INT> constexpr INT EXPONENT() const {
     if (Exponent() == maxExponent) {
       return INT::HUGE();
+    } else if (IsZero()) {
+      return {0};
     } else {
-      return {UnbiasedExponent()};
+      return {UnbiasedExponent() + 1};
     }
   }
 

 
   // Extracts unbiased exponent value.
   // Corrects the exponent value of a subnormal number.
+  // Note that the result is one less than the EXPONENT intrinsic;
+  // UnbiasedExponent(1.0) is 0, not 1.
   constexpr int UnbiasedExponent() const {
     int exponent{Exponent() - exponentBias};
     if (IsSubnormal()) {

flang/test/Evaluate/folding07.f90

   logical, parameter :: test_tiny10 = tiny10 == ztiny10
   logical, parameter :: test_tiny16 = tiny16 == ztiny16
 
+  logical, parameter :: test_exponent_0 = exponent(0.0) == 0
+  logical, parameter :: test_exponent_r8 = exponent(0.125) == -2
+  logical, parameter :: test_exponent_r4 = exponent(0.25) == -1
+  logical, parameter :: test_exponent_r2 = exponent(0.5) == 0
+  logical, parameter :: test_exponent_1 = exponent(1.0) == 1
+  logical, parameter :: test_exponent_4 = exponent(4.1) == 3
+  logical, parameter :: test_exponent_12 = exponent(12.9) == 4
+
   integer, parameter :: &
     max2 = maxexponent(0._2), &
     max3 = maxexponent(0._3), &