Make _grpc.h types C++20 compatible

[chwarr]

As of C++20, it is malformed to have an unnamed class used in typedef with a
base class. In the generated reflection code for a service, we were using the
following construct to create a reflectable entry for each field in a
struct.

struct service {
    typedef struct /* no name */ : ::bond::reflection::MethodTemplate<...> {} method_name;
}

Now, instead of using an unnamed class, we generate a named class and refer
to that:

struct service {
    typedef struct method_name_type : ::bond::reflection::MethodTemplate<...> {} field_name;
}

This commit is very similar in spirit to f1cb707. See that commit for
even more details about what's going on.

Also adds a test that compiles a service that has method names that
conflict with the names in MethodTemplate. The gRPC++ unit test CMake
now has a single library with the compiled generated code.

Fixes #1027

[chwarr]

@BertusGreeff, can you help review this? It's very similar to your PR #1028.

[chwarr]

This will probably need to be merged with #1039 to fix any AppVeyor build failures in the C# code.