Rename "umbrella" to "reflection" consistently.

This changes csharp_names.h, which will require a corresponding change in GRPC.
diff --git a/BUILD b/BUILD
index 7c09605..220e5c0 100644
--- a/BUILD
+++ b/BUILD
@@ -193,11 +193,11 @@
         "src/google/protobuf/compiler/csharp/csharp_message.cc",
         "src/google/protobuf/compiler/csharp/csharp_message_field.cc",
         "src/google/protobuf/compiler/csharp/csharp_primitive_field.cc",
+        "src/google/protobuf/compiler/csharp/csharp_reflection_class.cc",
         "src/google/protobuf/compiler/csharp/csharp_repeated_enum_field.cc",
         "src/google/protobuf/compiler/csharp/csharp_repeated_message_field.cc",
         "src/google/protobuf/compiler/csharp/csharp_repeated_primitive_field.cc",
         "src/google/protobuf/compiler/csharp/csharp_source_generator_base.cc",
-        "src/google/protobuf/compiler/csharp/csharp_umbrella_class.cc",
         "src/google/protobuf/compiler/csharp/csharp_wrapper_field.cc",
         "src/google/protobuf/compiler/java/java_context.cc",
         "src/google/protobuf/compiler/java/java_doc_comment.cc",