[flang] Add an entry to the intrinsics table for dimag.

Also, document extensions dconjg and dimag.

Original-commit: flang-compiler/f18@21535280ee
Reviewed-on: https://github.com/flang-compiler/f18/pull/654

flang/documentation/Extensions.md

@@ -94,6 +94,7 @@ Extensions, deletions, and legacy features supported by default
   we allow distinct types to be used, promoting
   the arguments as if they were operands to an intrinsic `+` operator,
   and defining the result type accordingly.
+* DOUBLE COMPLEX intrinsics DCONJG and DIMAG.
 
 Extensions supported when enabled by options
 --------------------------------------------

flang/lib/evaluate/intrinsics.cc

@@ -778,6 +778,8 @@ static const SpecificIntrinsicInterface specificIntrinsicFunction[]{
     {{"ddim", {{"x", DoublePrecision}, {"y", DoublePrecision}},
          DoublePrecision},
         "dim"},
+    {{"dimag", {{"a", DoublePrecisionComplex}}, DoublePrecision},
+        "aimag"},
     {{"dexp", {{"x", DoublePrecision}}, DoublePrecision}, "exp"},
     {{"dim", {{"x", DefaultReal}, {"y", DefaultReal}}, DefaultReal}},
     {{"dint", {{"a", DoublePrecision}}, DoublePrecision}, "aint"},