[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

               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;

flang/test/Semantics/case01.f90

   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