[flang] Disallow ASYNCHRONOUS for subroutine

[klausler]

Summary

The check for inappropriate usage of the ASYNCHRONOUS attribute
needed to be moved in declaration checking so that it can catch
attempts to use it on a subroutine.

[clementval]

LGTM

flang/lib/Semantics/check-declarations.cpp

void CheckHelper::Check(const Symbol &symbol) {
   if (symbol.attrs().test(Attr::SAVE) &&
       !symbol.implicitAttrs().test(Attr::SAVE)) {
     CheckExplicitSave(symbol);
   }
   if (symbol.attrs().test(Attr::CONTIGUOUS)) {
     CheckContiguous(symbol);
   }
   CheckGlobalName(symbol);
+  if (symbol.attrs().test(Attr::ASYNCHRONOUS) &&
+      !evaluate::IsVariable(symbol)) {
+    messages_.Say(
+        "An entity may not have the ASYNCHRONOUS attribute unless it is a variable"_err_en_US);
+  }
+
   if (isDone) {
     return; // following checks do not apply
   }
 
   if (symbol.attrs().test(Attr::PROTECTED)) {
     if (symbol.owner().kind() != Scope::Kind::Module) { // C854
       messages_.Say(
           "A PROTECTED entity must be in the specification part of a module"_err_en_US);
if (IsAllocatable(symbol)) {
           "Procedure '%s' may not be ALLOCATABLE without an explicit interface"_err_en_US,
           symbol.name());
     } else if (symbol.Rank() > 0) {
       messages_.Say(
           "Procedure '%s' may not be an array without an explicit interface"_err_en_US,
           symbol.name());
     }
   }
-  if (symbol.attrs().test(Attr::ASYNCHRONOUS) &&
-      !evaluate::IsVariable(symbol)) {
-    messages_.Say(
-        "An entity may not have the ASYNCHRONOUS attribute unless it is a variable"_err_en_US);
-  }
 }
 
 void CheckHelper::CheckCommonBlock(const Symbol &symbol) {
   CheckGlobalName(symbol);
   if (symbol.attrs().test(Attr::BIND_C)) {
     CheckBindC(symbol);
   }
 }

flang/test/Semantics/resolve20.f90

module m
   abstract interface
     function f()
     end function
   end interface
 
 contains
   subroutine bar
   end subroutine
+  !ERROR: An entity may not have the ASYNCHRONOUS attribute unless it is a variable
   subroutine test
+    asynchronous test
     !ERROR: Abstract procedure interface 'foo2' may not be referenced
     call foo2()
     !ERROR: Abstract procedure interface 'f' may not be referenced
     x = f()
   end subroutine
 end module