[flang] Downgrade benign error message to a warning

[klausler]

Summary

It's not conforming to specify the SAVE attribute more than
once for a variable, but it also doesn't hurt anything and
isn't fatal in other Fortran compilers. Downgrade the
message to a warning for better portability.

[PeteSteinfeld]

All builds, tests, and looks good.

flang/docs/Extensions.md

* We support some combinations of specific procedures in generic
   when their calls must nonetheless be distinguishable.
   Specifically, `ALLOCATABLE` dummy arguments are distinguishing
   if an actual argument acceptable to one could not be passed to
   the other & vice versa because exactly one is polymorphic or
   exactly one is unlimited polymorphic).
 * External unit 0 is predefined and connected to the standard error output,
   and defined as `ERROR_UNIT` in the intrinsic `ISO_FORTRAN_ENV` module.
 * Objects in blank COMMON may be initialized.
+* Multiple specifications of the SAVE attribute on the same object
+  are allowed, with a warning.
 
 ### Extensions supported when enabled by options
 
 * C-style backslash escape sequences in quoted CHARACTER literals
   (but not Hollerith) [-fbackslash]
 * Logical abbreviations `.T.`, `.F.`, `.N.`, `.A.`, `.O.`, and `.X.`
   [-flogical-abbreviations]
 * `.XOR.` as a synonym for `.NEQV.` [-fxor-operator]

flang/lib/Semantics/resolve-names.cpp

Attrs DeclarationVisitor::HandleSaveName(const SourceName &name, Attrs attrs) {
   return attrs;
 }
 
 // Record a name in a set of those to be saved.
 void DeclarationVisitor::AddSaveName(
     std::set<SourceName> &set, const SourceName &name) {
   auto pair{set.insert(name)};
   if (!pair.second) {
-    Say2(name, "SAVE attribute was already specified on '%s'"_err_en_US,
+    Say2(name, "SAVE attribute was already specified on '%s'"_en_US,
         *pair.first, "Previous specification of SAVE attribute"_en_US);
   }
 }
 
 // Set the SAVE attribute on symbol unless it is implicitly saved anyway.
 void DeclarationVisitor::SetSaveAttr(Symbol &symbol) {
   if (!IsSaved(symbol)) {
     symbol.attrs().set(Attr::SAVE);

flang/test/Semantics/resolve107.f90

+! RUN: %flang_fc1 -fsyntax-only %s 2>&1 | FileCheck %s
+! Check warning on multiple SAVE attribute specifications
+subroutine saves
+  save x
+  save y
+  !CHECK: SAVE attribute was already specified on 'y'
+  integer, save :: y
+  integer, save :: z
+  !CHECK: SAVE attribute was already specified on 'x'
+  !CHECK: SAVE attribute was already specified on 'z'
+  save x,z
+end
+

flang/test/Semantics/resolve45.f90

subroutine s5
   implicit none
   integer x
   block
     !ERROR: No explicit type declared for 'x'
     save x
   end block
 end
 
-subroutine s6
-  save x
-  save y
-  !ERROR: SAVE attribute was already specified on 'y'
-  integer, save :: y
-  integer, save :: z
-  !ERROR: SAVE attribute was already specified on 'x'
-  !ERROR: SAVE attribute was already specified on 'z'
-  save x,z
-end
-
 subroutine s7
   !ERROR: 'x' appears as a COMMON block in a SAVE statement but not in a COMMON statement
   save /x/
 end
 
 subroutine s8a(n)
   integer :: n
   real :: x(n)  ! OK: save statement doesn't affect x
   save
 end
 subroutine s8b(n)
   integer :: n
   !ERROR: SAVE attribute may not be applied to automatic data object 'x'
   real, save :: x(n)
 end