IR: Add MDNode::isDistinct()

Add API to indicate whether an `MDNode` is distinct.  A distinct node is
not stored in the MDNode uniquing tables, and will never be returned by
`MDNode::get()`.

Although distinct nodes are only currently created by uniquing
collisions (when operands change), PR22111 will allow these nodes to be
explicitly created.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225401 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Metadata.h

@@ -642,6 +642,12 @@ public:
   /// \brief Check if node is fully resolved.
   bool isResolved() const;
 
+  /// \brief Check if node is distinct.
+  ///
+  /// Distinct nodes are not uniqued, and will not be returned by \a
+  /// MDNode::get().
+  bool isDistinct() const { return IsDistinctInContext; }
+
 protected:
   /// \brief Set an operand.
   ///