[flang] Allow derf as alternate spelling for erf

This patch adds derf as an alternate spelling for the erf intrinsic.
This spelling is supported by multiple other compilers and used by WRF.

flang/lib/Evaluate/intrinsics.cpp

@@ -1098,6 +1098,7 @@ static const SpecificIntrinsicInterface specificIntrinsicFunction[]{
     {{"ddim", {{"x", DoublePrecision}, {"y", DoublePrecision}},
          DoublePrecision},
         "dim"},
+    {{"derf", {{"x", DoublePrecision}}, DoublePrecision}, "erf"},
     {{"dexp", {{"x", DoublePrecision}}, DoublePrecision}, "exp"},
     {{"dfloat", {{"a", AnyInt}}, DoublePrecision}, "real", true},
     {{"dim", {{"x", DefaultReal}, {"y", DefaultReal}}, DefaultReal}},

flang/test/Intrinsics/math-codegen.fir

@@ -862,6 +862,7 @@ func.func @_QPtest_real8(%arg0: !fir.ref<f64> {fir.bindc_name = "x"}) -> f64 {
 func.func private @erff(f32) -> f32
 func.func private @erf(f64) -> f64
 
+
 //--- exp_fast.fir
 // RUN: fir-opt %t/exp_fast.fir --fir-to-llvm-ir="target=x86_64-unknown-linux-gnu" | FileCheck %t/exp_fast.fir
 // CHECK: @_QPtest_real4

flang/unittests/Evaluate/intrinsics.cpp

@@ -344,6 +344,7 @@ void TestIntrinsics() {
   TEST(table.GetGenericIntrinsicName("dcos") == "cos");
   TEST(table.GetGenericIntrinsicName("dcosh") == "cosh");
   TEST(table.GetGenericIntrinsicName("ddim") == "dim");
+  TEST(table.GetGenericIntrinsicName("derf") == "erf");
   TEST(table.GetGenericIntrinsicName("dexp") == "exp");
   TEST(table.GetGenericIntrinsicName("dint") == "aint");
   TEST(table.GetGenericIntrinsicName("dlog") == "log");