[flang] Improve message for assignment to subprogram

In the example below we were producing the error message
"Assignment to constant 'f' is not allowed":
```
function f() result(r)
  f = 1.0
end
```

This changes it to a more helpful message when the LHS is a subprogram
name and also mentions the function result name when it's a function.

Differential Revision: https://reviews.llvm.org/D85483

flang/lib/Semantics/expression.cpp

@@ -2704,10 +2704,22 @@ void ArgumentAnalyzer::Analyze(const parser::Variable &x) {
       actuals_.emplace_back(std::move(*expr));
       return;
     }
-    const Symbol *symbol{GetFirstSymbol(*expr)};
-    context_.Say(x.GetSource(),
-        "Assignment to constant '%s' is not allowed"_err_en_US,
-        symbol ? symbol->name() : x.GetSource());
+    const Symbol *symbol{GetLastSymbol(*expr)};
+    if (!symbol) {
+      context_.SayAt(x, "Assignment to constant '%s' is not allowed"_err_en_US,
+          x.GetSource());
+    } else if (auto *subp{symbol->detailsIf<semantics::SubprogramDetails>()}) {
+      auto *msg{context_.SayAt(x,
+          "Assignment to subprogram '%s' is not allowed"_err_en_US,
+          symbol->name())};
+      if (subp->isFunction()) {
+        const auto &result{subp->result().name()};
+        msg->Attach(result, "Function result is '%s'"_err_en_US, result);
+      }
+    } else {
+      context_.SayAt(x, "Assignment to constant '%s' is not allowed"_err_en_US,
+          symbol->name());
+    }
   }
   fatalErrors_ = true;
 }

flang/test/Semantics/assign04.f90

@@ -115,3 +115,13 @@ subroutine s7
   integer :: a(10), v(10)
   a(v(:)) = 1  ! vector subscript is ok
 end
+
+subroutine s8
+  !ERROR: Assignment to subprogram 's8' is not allowed
+  s8 = 1.0
+end
+
+real function f9() result(r)
+  !ERROR: Assignment to subprogram 'f9' is not allowed
+  f9 = 1.0
+end