[flang][openacc] Fix ambiguity in the self clause parsing

[clementval]

Summary

In the OpenACC specification, there are two different self clause. One for the
update directive with a var-list argument. This clause is a synonym of the host clause.
The second self clause is present for most of the compute construct and takes an optional
condition. To solve this ambiguity, the self clause for the update directive is directly
translated to a host clause during the parsing. The self clause in AccClause refers always
to the compute construct clause.

[kiranktp]

LGTM

flang/lib/Parser/openacc-parsers.cpp

"PRESENT" >> construct<AccClause>(construct<AccClause::Present>(
                      parenthesized(Parser<AccObjectList>{}))) ||
     "PRIVATE" >> construct<AccClause>(construct<AccClause::Private>(
                      parenthesized(Parser<AccObjectList>{}))) ||
     "READ" >> construct<AccClause>(construct<AccClause::Read>()) ||
     "REDUCTION" >> construct<AccClause>(construct<AccClause::Reduction>(
                        parenthesized(construct<AccObjectListWithReduction>(
                            Parser<AccReductionOperator>{} / ":",
                            Parser<AccObjectList>{})))) ||
+    // SELF clause is either a simple optional condition for compute construct
+    // or a synonym of the HOST clause for the update directive 2.14.4 holding
+    // an object list.
     "SELF" >> construct<AccClause>(construct<AccClause::Self>(
                   maybe(parenthesized(scalarLogicalExpr)))) ||
+    construct<AccClause>(
+        construct<AccClause::Host>(parenthesized(Parser<AccObjectList>{}))) ||
     "SEQ" >> construct<AccClause>(construct<AccClause::Seq>()) ||
     "TILE" >> construct<AccClause>(construct<AccClause::Tile>(
                   parenthesized(Parser<AccTileExprList>{}))) ||
     "USE_DEVICE" >> construct<AccClause>(construct<AccClause::UseDevice>(
                         parenthesized(Parser<AccObjectList>{}))) ||
     "VECTOR_LENGTH" >> construct<AccClause>(construct<AccClause::VectorLength>(
                            parenthesized(scalarIntExpr))) ||
     "VECTOR" >>

flang/test/Semantics/acc-clause-validity.f90

program openacc_clause_validity
   logical, dimension(N) :: d, e
   real :: reduction_r
   logical :: reduction_l
   real(8), dimension(N, N) :: aa
   logical :: ifCondition = .TRUE.
 
   !ERROR: At least one clause is required on the DECLARE directive
   !$acc declare
-  real(8), dimension(N) :: a
+  real(8), dimension(N) :: a, f, g, h
 
   !$acc init
   !$acc init if(.TRUE.)
   !$acc init if(ifCondition)
   !$acc init device_num(1)
   !$acc init device_num(i)
   !$acc init device_type(i)
   !$acc init device_type(2, i, j)
program openacc_clause_validity
 
   !$acc data copyin(i)
   !$acc end data
 
   !$acc data copyin(i)
   !ERROR: Unmatched PARALLEL directive
   !$acc end parallel
 
+  !$acc update self(a, f) host(g) device(h)
+
   !$acc update device(i) device_type(*) async
 
   !ERROR: Clause IF is not allowed after clause DEVICE_TYPE on the UPDATE directive
   !$acc update device(i) device_type(*) if(.TRUE.)
 
   !$acc parallel
   !ERROR: INDEPENDENT and SEQ clauses are mutually exclusive and may not appear on the same LOOP directive
   !$acc loop seq independent
program openacc_clause_validity
   end do
   !$acc end parallel
 
   !$acc parallel loop tile(2)
   do i = 1, N
     a(i) = 3.14
   end do
 
+  !$acc parallel loop self
+  do i = 1, N
+    a(i) = 3.14
+  end do
+
+  !$acc parallel loop self(.true.)
+  do i = 1, N
+    a(i) = 3.14
+  end do
+
+  !$acc parallel loop self(ifCondition)
+  do i = 1, N
+    a(i) = 3.14
+  end do
+
   !$acc parallel loop tile(2, 2)
   do i = 1, N
     do j = 1, N
       aa(i, j) = 3.14
     end do
   end do
 
   !$acc parallel device_type(*) num_gangs(2)