[flang] Downgrade a fatal error to a warning

[klausler]

Summary

A non-CHARACTER expression in a CASE statement is allowed to have
a distinct kind (not type) from the expression in its SELECT CASE.
If a value in a CASE statement is out of range for the SELECT CASE
type, emit a warning, but it should not be a fatal error.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Semantics/check-case.cpp

if (x && x->v) { // C1147
           if (auto value{evaluate::GetScalarConstantValue<T>(*converted)}) {
             auto back{evaluate::Fold(foldingContext,
                 evaluate::ConvertToType(*type, SomeExpr{*converted}))};
             if (back == folded) {
               x->v = converted;
               return value;
             } else {
               context_.Say(expr.source,
-                  "CASE value (%s) overflows type (%s) of SELECT CASE expression"_err_en_US,
+                  "CASE value (%s) overflows type (%s) of SELECT CASE expression"_warn_en_US,
                   folded.AsFortran(), caseExprType_.AsFortran());
               hasErrors_ = true;
               return std::nullopt;
             }
           }
         }
         context_.Say(expr.source,
             "CASE value (%s) must be a constant scalar"_err_en_US,

flang/test/Semantics/case01.f90

select case(i)
     case(:0)
   end select
 end
 
 program test_overflow
   integer :: j
   select case(1_1)
   case (127)
-  !ERROR: CASE value (128_4) overflows type (INTEGER(1)) of SELECT CASE expression
+  !WARN: CASE value (128_4) overflows type (INTEGER(1)) of SELECT CASE expression
   case (128)
-  !ERROR: CASE value (129_4) overflows type (INTEGER(1)) of SELECT CASE expression
-  !ERROR: CASE value (130_4) overflows type (INTEGER(1)) of SELECT CASE expression
+  !WARN: CASE value (129_4) overflows type (INTEGER(1)) of SELECT CASE expression
+  !WARN: CASE value (130_4) overflows type (INTEGER(1)) of SELECT CASE expression
   case (129:130)
-  !ERROR: CASE value (-130_4) overflows type (INTEGER(1)) of SELECT CASE expression
-  !ERROR: CASE value (-129_4) overflows type (INTEGER(1)) of SELECT CASE expression
+  !WARN: CASE value (-130_4) overflows type (INTEGER(1)) of SELECT CASE expression
+  !WARN: CASE value (-129_4) overflows type (INTEGER(1)) of SELECT CASE expression
   case (-130:-129)
   case (-128)
   !ERROR: Must be a scalar value, but is a rank-1 array
   case ([1, 2])
   !ERROR: Must be a constant value
   case (j)
   case default
   end select
 end