[flang][OpenMP] Don't allow DO CONCURRENT inside of a loop nest

I don't think DO CONCURRENT fits the definition of a Canonical Loop Nest
(OpenMP 6.0 section 6.4.1).

Fixes #144178

Author: tblah

flang/lib/Semantics/resolve-directives.cpp

@@ -1952,6 +1952,12 @@ void OmpAttributeVisitor::PrivatizeAssociatedLoopIndexAndCheckLoopLevel(
   const auto &outer{std::get<std::optional<parser::DoConstruct>>(x.t)};
   if (outer.has_value()) {
     for (const parser::DoConstruct *loop{&*outer}; loop && level > 0; --level) {
+      if (loop->IsDoConcurrent()) {
+        auto &stmt =
+            std::get<parser::Statement<parser::NonLabelDoStmt>>(loop->t);
+        context_.Say(stmt.source,
+            "DO CONCURRENT loops cannot form part of a loop nest."_err_en_US);
+      }
       // go through all the nested do-loops and resolve index variables
       const parser::Name *iv{GetLoopIndex(*loop)};
       if (iv) {

flang/test/Lower/OpenMP/Todo/omp-doconcurrent.f90

@@ -0,0 +1,19 @@
+!RUN: %python %S/../test_errors.py %s %flang -fopenmp
+
+integer :: i, j
+!$omp parallel do collapse(2)
+do i = 1, 1
+  ! ERROR: DO CONCURRENT loops cannot form part of a loop nest.
+  do concurrent (j = 1:2)
+    print *, j
+  end do
+end do
+
+!$omp parallel do
+do i = 1, 1
+  ! This should not lead to an error because it is not part of a loop nest:
+  do concurrent (j = 1:2)
+    print *, j
+  end do
+end do
+end