[flang][openacc] Relax clause rule on routine directive

[clementval]

Summary

Some compiler treat acc routine without a parallelism clause as
if seq is present. Relax the parser rule to allow acc routine
without clause. The default clause will be handled in lowering.

[razvanlupusoru]

This part looks great.

flang/docs/OpenACC.md

 
 ```eval_rst
 .. contents::
    :local:
 ```
 
 ## Intentional deviation from the specification
 * The end directive for combined construct can omit the `loop` keyword.
+* An `!$acc routine` with no parallelism clause is treated as if the `seq`
+  clause was present.

flang/test/Semantics/OpenACC/acc-routine-validity.f90

 ! RUN: %python %S/../test_errors.py %s %flang -fopenacc
 
 ! Check OpenACC clause validity for the following construct and directive:
 !   2.15.1 routine
 
 module openacc_routine_validity
   implicit none
 
   !$acc routine(sub3) seq
 
   !$acc routine(fct2) vector
 
-  !ERROR: At least one of GANG, SEQ, VECTOR, WORKER clause must appear on the ROUTINE directive
   !$acc routine(sub3)
 
   !ERROR: ROUTINE directive without name must appear within the specification part of a subroutine or function definition, or within an interface body for a subroutine or function in an interface block
   !$acc routine seq
 
   !ERROR: No function or subroutine declared for 'dummy'
   !$acc routine(dummy) seq
 
 contains
 
   subroutine sub1(a)
     real :: a(:)
-    !ERROR: At least one of GANG, SEQ, VECTOR, WORKER clause must appear on the ROUTINE directive
     !$acc routine
   end subroutine sub1
 
   subroutine sub2(a)
     real :: a(:)
     !ERROR: Clause NOHOST is not allowed after clause DEVICE_TYPE on the ROUTINE directive
     !$acc routine seq device_type(*) nohost
   end subroutine sub2

llvm/include/llvm/Frontend/OpenACC/ACC.td

def ACC_Init : Directive<"init"> {
   ];
 }
 
 // 2.15.1
 def ACC_Routine : Directive<"routine"> {
   let allowedOnceClauses = [
     VersionedClause<ACCC_Bind>,
     VersionedClause<ACCC_DeviceType>,
-    VersionedClause<ACCC_NoHost>
-  ];
-  let requiredClauses = [
+    VersionedClause<ACCC_NoHost>,
     VersionedClause<ACCC_Gang>,
     VersionedClause<ACCC_Seq>,
     VersionedClause<ACCC_Vector>,
     VersionedClause<ACCC_Worker>
   ];
 }
 
 // 2.14.3