[flang] Check C854, C855, & C856 on PROTECTED entities

[klausler]

Summary

Check for things that are not allowed to bear the PROTECTED
attribute.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Semantics/check-declarations.cpp

   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 (!evaluate::IsVariable(symbol) && !IsProcedurePointer(symbol)) { // C855
+      messages_.Say(
+          "A PROTECTED entity must be a variable or pointer"_err_en_US);
+    }
+    if (InCommonBlock(symbol)) { // C856
+      messages_.Say(
+          "A PROTECTED entity may not be in a common block"_err_en_US);
+    }
+  }
   if (IsPointer(symbol)) {
     CheckPointer(symbol);
   }

flang/test/Semantics/resolve82.f90

   procedure(procFunc), bind(c), pointer, bind(c) :: proc3
   !WARNING: Attribute 'PROTECTED' cannot be used more than once
   procedure(procFunc), protected, pointer, protected :: proc4
+  !ERROR: A PROTECTED entity must be a variable or pointer
+  external extsub
+  protected extsub
+  real x
+  !ERROR: A PROTECTED entity must be a variable or pointer
+  namelist /nml/ x
+  protected nml
+  !ERROR: A PROTECTED entity may not be in a common block
+  real y
+  common /blk/ y
+  protected y
 
 contains
 

       procedure(procFunc), pointer, optional, pointer :: arg10
       !WARNING: Attribute 'SAVE' cannot be used more than once
       procedure(procFunc), save, pointer, save :: localProc
+      !ERROR: A PROTECTED entity must be in the specification part of a module
+      real x
+      protected x
     end subroutine testProcDecl
 
 end module m