[flang] Improve error message (initialized variable in pure subprogram)

[klausler]

Summary

When variable with the SAVE attribute appears in a pure subprogram,
emit a more specialized error message if the SAVE attribute was acquired
from static initialization.

flang/lib/Semantics/check-declarations.cpp

void CheckHelper::Check(const Symbol &symbol) {
   if (isDone) {
     return; // following checks do not apply
   }
   if (IsPointer(symbol)) {
     CheckPointer(symbol);
   }
   if (InPure()) {
     if (IsSaved(symbol)) {
+      if (IsInitialized(symbol)) {
+        messages_.Say(
+            "A pure subprogram may not initialize a variable"_err_en_US);
+      } else {
         messages_.Say(
             "A pure subprogram may not have a variable with the SAVE attribute"_err_en_US);
       }
+    }
     if (symbol.attrs().test(Attr::VOLATILE)) {
       messages_.Say(
           "A pure subprogram may not have a variable with the VOLATILE attribute"_err_en_US);
     }
     if (IsProcedure(symbol) && !IsPureProcedure(symbol) && IsDummy(symbol)) {
       messages_.Say(
           "A dummy procedure of a pure subprogram must be pure"_err_en_US);
     }

flang/test/Semantics/call10.f90

contains
   end subroutine
   pure subroutine s04(a) ! C1588
     !ERROR: An INTENT(OUT) dummy argument of a pure subroutine may not have a polymorphic ultimate component
     type(polyAlloc), intent(out) :: a
   end subroutine
   pure subroutine s05 ! C1589
     !ERROR: A pure subprogram may not have a variable with the SAVE attribute
     real, save :: v1
-    !ERROR: A pure subprogram may not have a variable with the SAVE attribute
+    !ERROR: A pure subprogram may not initialize a variable
     real :: v2 = 0.
-    !ERROR: A pure subprogram may not have a variable with the SAVE attribute
+    !ERROR: A pure subprogram may not initialize a variable
     real :: v3
     data v3/0./
     !ERROR: A pure subprogram may not have a variable with the SAVE attribute
     real :: v4
     common /blk/ v4
     save /blk/
     block
     !ERROR: A pure subprogram may not have a variable with the SAVE attribute
       real, save :: v5
-    !ERROR: A pure subprogram may not have a variable with the SAVE attribute
+    !ERROR: A pure subprogram may not initialize a variable
       real :: v6 = 0.
     end block
   end subroutine
   pure subroutine s06 ! C1589
     !ERROR: A pure subprogram may not have a variable with the VOLATILE attribute
     real, volatile :: v1
     block
     !ERROR: A pure subprogram may not have a variable with the VOLATILE attribute