[Frontend][OpenMP] Allow implicit clauses to fail to apply (llvm#100460)

The `linear(x)` clause implies `firstprivate(x)` on the compound
construct if `x` is not an induction variable. With more construct
combinations coming in OpenMP 6.0, the `firstprivate` clause may not be
possible to apply, e.g. in "masked simd".
An additional benefit from this change is that it allows treating leaf
constructs as combined constructs with a single constituent. Otherwise,
a `linear` clause on a lone `simd` construct could imply a
`firstprivate` clause that can't be applied.

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

@@ -1114,6 +1114,11 @@ bool ConstructDecompositionT<C, H>::applyClause(
 template <typename C, typename H> bool ConstructDecompositionT<C, H>::split() {
   bool success = true;
 
+  auto isImplicit = [this](const ClauseTy *node) {
+    return llvm::any_of(
+        implicit, [node](const ClauseTy &clause) { return &clause == node; });
+  };
+
   for (llvm::omp::Directive leaf :
        llvm::omp::getLeafConstructsOrSelf(construct))
     leafs.push_back(LeafReprInternal{leaf, /*clauses=*/{}});
@@ -1153,9 +1158,10 @@ template <typename C, typename H> bool ConstructDecompositionT<C, H>::split() {
   for (const ClauseTy *node : nodes) {
     if (skip(node))
       continue;
-    success =
-        success &&
+    bool result =
         std::visit([&](auto &&s) { return applyClause(s, node); }, node->u);
+    if (!isImplicit(node))
+      success = success && result;
   }
 
   // Apply "allocate".

llvm/unittests/Frontend/OpenMPDecompositionTest.cpp

@@ -1100,4 +1100,21 @@ TEST_F(OpenMPDecompositionTest, Nowait1) {
   ASSERT_EQ(Dir1, "parallel");      // (23)
   ASSERT_EQ(Dir2, "for");           // (23)
 }
+
+// ---
+
+// Check that "simd linear(x)" does not fail despite the implied "firstprivate"
+// (which "simd" does not allow).
+TEST_F(OpenMPDecompositionTest, Misc1) {
+  omp::Object x{"x"};
+  omp::List<omp::Clause> Clauses{
+      {OMPC_linear,
+       omp::clause::Linear{{std::nullopt, std::nullopt, std::nullopt, {x}}}},
+  };
+
+  omp::ConstructDecomposition Dec(AnyVersion, Helper, OMPD_simd, Clauses);
+  ASSERT_EQ(Dec.output.size(), 1u);
+  std::string Dir0 = stringify(Dec.output[0]);
+  ASSERT_EQ(Dir0, "simd linear(, , , (x)) lastprivate(, (x))");
+}
 } // namespace