[OpenMP] Apply COLLAPSE to innermost leaf that allows it (llvm#167565)

As per the wording from 5.2, the COLLAPSE clause applies once to the
entire construct. The 6.0 spec has a somewhat similar wording with the
same intent. In practice, apply the clause to the innermost leaf
constituent that allows it, without requiring it to be the exact
innermost leaf.

Author: kparzysz

llvm/include/llvm/Frontend/OpenMP/ConstructDecompositionT.h

@@ -501,18 +501,7 @@ template <typename C, typename H>
 bool ConstructDecompositionT<C, H>::applyClause(
     const tomp::clause::CollapseT<TypeTy, IdTy, ExprTy> &clause,
     const ClauseTy *node) {
-  // Apply "collapse" to the innermost directive. If it's not one that
-  // allows it flag an error.
-  if (!leafs.empty()) {
-    auto &last = leafs.back();
-
-    if (llvm::omp::isAllowedClauseForDirective(last.id, node->id, version)) {
-      last.clauses.push_back(node);
-      return true;
-    }
-  }
-
-  return false;
+  return applyToInnermost(node);
 }
 
 // DEFAULT