hdl._ir: chain Fragment.origins with preexisting

[rroohhh]

This PR changes Fragment.origins from being overwritten if during Fragment.get a Fragment is encountered that already has a Fragment.origins field to
appending to the preexisting origins tuple. This case is interesting for anything using a FragmentTransformer. With this change,

from amaranth import *

for transformer in (DomainRenamer("sync"), ResetInserter(Signal()), EnableInserter(Signal())):
    origins = Fragment.get(transformer(inst := Module()), None).origins
    assert inst in origins

no longer asserts, like it does currently.

Some before after examples:

Before:

>>> Fragment.get(DomainRenamer("sync")(Module()), None).origins
(<amaranth.hdl._xfrm.TransformedElaboratable object at 0x7fa86fee1d10>,)
>>> Fragment.get(ResetInserter(Signal())(DomainRenamer("sync")(Module())), None).origins
(<amaranth.hdl._xfrm.TransformedElaboratable object at 0x7fa86fee3310>,)

After:

>>> Fragment.get(DomainRenamer("sync")(Module()), None).origins
(<amaranth.hdl._xfrm.TransformedElaboratable object at 0x7f701f7266d0>, <amaranth.hdl._dsl.Module object at 0x7f70200a6690>)
>>> Fragment.get(ResetInserter(Signal())(DomainRenamer("sync")(Module())), None).origins
(<amaranth.hdl._xfrm.TransformedElaboratable object at 0x7f701f425a50>, <amaranth.hdl._dsl.Module object at 0x7f701f8f9250>)

Furthermore, with this change, the duplicate fragment check performed by Fragment.prepare() now also works if the duplicate is behind something that uses a fragment transformer, for example

from amaranth import *

m = Module()
m.submodules.a1 = a1 = DomainRenamer("sync")(inst := Module())
m.submodules.a2 = a2 = inst

Fragment.get(m, None).prepare()

previously did not raise any exception, but now (correctly) raises:

amaranth.hdl._ir.DuplicateElaboratable: Elaboratable <amaranth.hdl._dsl.Module object at 0x7fabfb3872d0> is included twice in the hierarchy, as top.a1 and top.a2

[whitequark]

LGTM, thanks!