Rename GeneratedCodeInfo to GeneratedClrTypeInfo

Recently, descriptor.proto gained a GeneratedCodeInfo message, which means the generated code conflicts with our type.
Unfortunately this affects codegen as well, although this is a part of the public API which is very unlikely to affect hand-written code.

Generated code changes in next commit.
diff --git a/Makefile.am b/Makefile.am
index 188346f..c7267e0 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -146,7 +146,7 @@
   csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs                   \
   csharp/src/Google.Protobuf/Reflection/FieldType.cs                         \
   csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs                    \
-  csharp/src/Google.Protobuf/Reflection/GeneratedCodeInfo.cs                 \
+  csharp/src/Google.Protobuf/Reflection/GeneratedClrTypeInfo.cs              \
   csharp/src/Google.Protobuf/Reflection/IDescriptor.cs                       \
   csharp/src/Google.Protobuf/Reflection/IFieldAccessor.cs                    \
   csharp/src/Google.Protobuf/Reflection/MapFieldAccessor.cs                  \