Propagate types from symbol to aliases.

This is similar, but not identical to what gas does. The logic in MC is to just
compute the symbol table after parsing the entire file. GAS is mixed, given

.type b, @object
a = b
b:
.type b, @function

It will propagate the change and make 'a' a function. Given

.type b, @object
b:
a = b
.type b, @function

the type of 'a' is still object.

Since we do the computation in the end, we produce a function in both cases.