[flang][OpenMP] Add comments to IsContiguous, improve formatting, NFC

Author: kparzysz

flang/lib/Semantics/check-omp-structure.cpp

@@ -302,22 +302,26 @@ struct ContiguousHelper {
 };
 } // namespace
 
+// Return values:
+// - std::optional<bool>{true} if the object is known to be contiguous
+// - std::optional<bool>{false} if the object is known not to be contiguous
+// - std::nullopt if the object contiguity cannot be determined
 std::optional<bool> OmpStructureChecker::IsContiguous(
     const parser::OmpObject &object) {
-  return common::visit(common::visitors{
-                           [&](const parser::Name &x) {
-                             // Any member of a common block must be contiguous.
-                             return std::optional<bool>{true};
-                           },
-                           [&](const parser::Designator &x) {
-                             evaluate::ExpressionAnalyzer ea{context_};
-                             if (MaybeExpr maybeExpr{ea.Analyze(x)}) {
-                               return ContiguousHelper{context_}.Visit(
-                                   *maybeExpr);
-                             }
-                             return std::optional<bool>{};
-                           },
-                       },
+  return common::visit( //
+      common::visitors{
+          [&](const parser::Name &x) {
+            // Any member of a common block must be contiguous.
+            return std::optional<bool>{true};
+          },
+          [&](const parser::Designator &x) {
+            evaluate::ExpressionAnalyzer ea{context_};
+            if (MaybeExpr maybeExpr{ea.Analyze(x)}) {
+              return ContiguousHelper{context_}.Visit(*maybeExpr);
+            }
+            return std::optional<bool>{};
+          },
+      },
       object.u);
 }