Go: add tests regarding type aliasing

[smowton]

Targeted at rc/3.15 because we intend to get Go 1.23 into this branch, and the Go 1.23 compiler exposes us to type aliasing considerably more than 1.22.

Suggested additional tests:

• Dataflow via field/pointer aliasing (sketch: type T1 = struct { x int }; type T2 = struct { x IntAlias } ... var t1 T1 ... t1.x = source() ... f(&t1) ... func f(t2 *T2) { sink(t2.x) })
• Dataflow via field/pointer aliasing including embedding -- similar to the above, but exercise the case where embedding is done via type aliasing -- so e.g. struct { Embedded1 } vs struct { Embedded2 }, where type Embedded2 = Embedded1. This needed particularly handling at df13f89
• Dataflow across interface implementations, an extension of the interface-implements test already committed here to ensure the dataflow lib's idea of interface implementation matches Type.implements.

Update: those additional tests are added in #17400