Add FrozenError#receiver

Similar to NameError#receiver, this returns the object on which
the modification was attempted.  This is useful as it can pinpoint
exactly what is frozen.  In many cases when a FrozenError is
raised, you cannot determine from the context which object is
frozen that you attempted to modify.

Users of the current rb_error_frozen C function will have to switch
to using rb_error_frozen_object or the new rb_frozen_error_raise
in order to set the receiver of the FrozenError.

To allow the receiver to be set from Ruby, support an optional
second argument to FrozenError#initialize.

Implements [Feature #15751]

spec/ruby/core/exception/frozen_error_spec.rb

@@ -0,0 +1,34 @@
+require_relative '../../spec_helper'
+
+describe "FrozenError" do
+  ruby_version_is "2.5" do
+    it "is a subclass of RuntimeError" do
+      RuntimeError.should be_ancestor_of(FrozenError)
+    end
+  end
+end
+
+describe "FrozenError.new" do
+  ruby_version_is "2.7" do
+    it "should take optional receiver argument" do
+      o = Object.new
+      FrozenError.new("msg", o).receiver.should equal(o)
+    end
+  end
+end
+
+describe "FrozenError#receiver" do
+  ruby_version_is "2.7" do
+    it "should return frozen object that modification was attempted on" do
+      o = Object.new.freeze
+      begin
+        def o.x; end
+      rescue => e
+        e.should be_kind_of(FrozenError)
+        e.receiver.should equal(o)
+      else
+        raise
+      end
+    end
+  end
+end