[flang] Catch attempt to apply ASYNCHRONOUS attribute to a non-variable

[klausler]

Summary

Complain about the ASYNCHRONOUS attribute on symbols that aren't
variables.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Semantics/check-declarations.cpp

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
   external :: a, b, c, d
   !ERROR: EXTERNAL attribute not allowed on 'm'
   external :: m
   !WARNING: EXTERNAL attribute was already specified on 'foo'
   external :: foo
   !ERROR: EXTERNAL attribute not allowed on 'bar'
   external :: bar
 
+  !ERROR: An entity may not have the ASYNCHRONOUS attribute unless it is a variable
+  asynchronous :: async
+  external :: async
+
   !ERROR: PARAMETER attribute not allowed on 'm'
   parameter(m=2)
   !ERROR: PARAMETER attribute not allowed on 'foo'
   parameter(foo=2)
   !ERROR: PARAMETER attribute not allowed on 'bar'
   parameter(bar=2)
 
   type, abstract :: t1