[mlir] Do not expose MLIRContext::isDialectLoading

[springerm]

Summary

This addresses post-commit comments and should have been part of D136685.

mlir/include/mlir/IR/MLIRContext.h

public:
   T *getOrLoadDialect() {
     return static_cast<T *>(
         getOrLoadDialect(T::getDialectNamespace(), TypeID::get<T>(), [this]() {
           std::unique_ptr<T> dialect(new T(this));
           return dialect;
         }));
   }
 
-  /// Return true if the given dialect is currently loading.
-  bool isDialectLoading(StringRef dialectNamespace);
-
   /// Load a dialect in the context.
   template <typename Dialect>
   void loadDialect() {
     // Do not load the dialect if it is currently loading. This can happen if a
     // dialect initializer triggers loading the same dialect recursively.
     if (!isDialectLoading(Dialect::getDialectNamespace()))
       getOrLoadDialect<Dialect>();
   }
public:
 
   /// Returns a hash of the registry of the context that may be used to give
   /// a rough indicator of if the state of the context registry has changed. The
   /// context registry correlates to loaded dialects and their entities
   /// (attributes, operations, types, etc.).
   llvm::hash_code getRegistryHash();
 
 private:
+  /// Return true if the given dialect is currently loading.
+  bool isDialectLoading(StringRef dialectNamespace);
+
   const std::unique_ptr<MLIRContextImpl> impl;
 
   MLIRContext(const MLIRContext &) = delete;
   void operator=(const MLIRContext &) = delete;
 };
 
 //===----------------------------------------------------------------------===//
 // MLIRContext CommandLine Options

mlir/lib/IR/MLIRContext.cpp

 #ifndef NDEBUG
     if (impl.multiThreadedExecutionContext != 0)
       llvm::report_fatal_error(
           "Loading a dialect (" + dialectNamespace +
           ") while in a multi-threaded execution context (maybe "
           "the PassManager): this can indicate a "
           "missing `dependentDialects` in a pass for example.");
 #endif // NDEBUG
-    // nullptr indicates that the dialect is currently being loaded.
-    impl.loadedDialects[dialectNamespace] = nullptr;
-    std::unique_ptr<Dialect> &dialect = impl.loadedDialects[dialectNamespace] =
-        ctor();
+    // loadedDialects entry is initialized to nullptr, indicating that the
+    // dialect is currently being loaded.
+    std::unique_ptr<Dialect> &dialect = impl.loadedDialects[dialectNamespace];
+    dialect = ctor();
     assert(dialect && "dialect ctor failed");
 
     // Refresh all the identifiers dialect field, this catches cases where a
     // dialect may be loaded after identifier prefixed with this dialect name
     // were already created.
     auto stringAttrsIt = impl.dialectReferencingStrAttrs.find(dialectNamespace);
     if (stringAttrsIt != impl.dialectReferencingStrAttrs.end()) {
       for (StringAttrStorage *storage : stringAttrsIt->second)