[flang-rt] Added IsContiguousUpTo runtime function. (#131048)

I want to be able to check if the storage is contiguous
in the innermost dimension, so I decided to add an entry point
that takes `dim` as the number of leading dimensions to check.
It seems that a runtime call might result in less code size
even when `dim` is 1, so here it is.
For opt-for-speed I am going to inline it in FIR.

Depends on #131047.

flang-rt/lib/runtime/support.cpp

@@ -19,6 +19,10 @@ bool RTDEF(IsContiguous)(const Descriptor &descriptor) {
   return descriptor.IsContiguous();
 }
 
+bool RTDEF(IsContiguousUpTo)(const Descriptor &descriptor, int dim) {
+  return descriptor.IsContiguous(dim);
+}
+
 bool RTDEF(IsAssumedSize)(const Descriptor &descriptor) {
   return ISO::IsAssumedSize(&descriptor.raw());
 }

flang-rt/unittests/Runtime/Support.cpp

@@ -78,3 +78,23 @@ TEST(DescriptorBytesFor, Basic) {
     EXPECT_GT(b, 0U);
   }
 }
+
+TEST(IsContiguous, Basic) {
+  // ARRAY  1 3 5
+  //        2 4 6
+  auto array{MakeArray<TypeCategory::Integer, 4>(
+      std::vector<int>{2, 3}, std::vector<std::int32_t>{1, 2, 3, 4, 5, 6})};
+  StaticDescriptor<2> sectionStaticDesc;
+  Descriptor &section{sectionStaticDesc.descriptor()};
+  section.Establish(array->type(), array->ElementBytes(),
+      /*p=*/nullptr, /*rank=*/2);
+  static const SubscriptValue lbs[]{1, 1}, ubs[]{2, 3}, strides[]{1, 2};
+  const auto error{
+      CFI_section(&section.raw(), &array->raw(), lbs, ubs, strides)};
+  ASSERT_EQ(error, 0) << "CFI_section failed for array: " << error;
+
+  EXPECT_TRUE(RTNAME(IsContiguous)(*array));
+  EXPECT_FALSE(RTNAME(IsContiguous)(section));
+  EXPECT_TRUE(RTNAME(IsContiguousUpTo)(section, 1));
+  EXPECT_FALSE(RTNAME(IsContiguousUpTo)(section, 2));
+}