Refactor code revolving source objects such that most logics will be
reused.
The main change is to expose a single `CreateSource(addr, target)` that
can return either a normal or an assembly source object, and call
`ShouldDisplayAssemblySource()` only from this function instead of
multiple places across the code.
Other functions can use `source.IsAssemblySource()` in order to check
which type the source is.
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