Clarify order of well-known-type conversion vs. selection, indexing, and type check

[JimLarson]

The language spec doesn't clearly explain how the automatic conversion of dynamic value protos interacts with type checking and runtime selection/indexing. For instance, given the following proto definitions:

message FakeAny {
    bytes value = 2;
}

and the following declarations and bindings

a: google.protobuf.Int64Value = 7
b: google.protobuf.Any = {...encoding of FakeAny{ value: "\377\377\377" }...}

do the following expressions type check, and what do they evaluate to?

a + 3
a.int_value + 3
b.value
b.value.value

I believe that what's intended is that the first and third will execute without error yielding 10 and b'\377\377\377' respectively, while the second and fourth should fail.