passes/unusedwrites: Add TODO for how to handle generics.
passes/unusedwrites: Add TODO for how to handle generics.

Adds a TODO to explain how to support generics, e.g. fn._Instantiations() should be included once available.

Refactors the run() function to make this simple in the future.

Updates golang/go#52503

Change-Id: Iec84f9bf200cab1026b19e1962165102be0a85ef
Reviewed-on: https://go-review.googlesource.com/c/tools/+/403355
Reviewed-by: Zvonimir Pavlinovic <490af24f3def1258d2f7e9b10b20e341f6c68320@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Run-TryBot: Guodong Li <31ea89c7b27f6bb34660011ae239934428d36db2@google.com>
Run-TryBot: Tim King <ee8a1d1efb05d8171fc0140ced15d455e83941c5@google.com>
Reviewed-by: Guodong Li <31ea89c7b27f6bb34660011ae239934428d36db2@google.com>
Auto-Submit: Tim King <ee8a1d1efb05d8171fc0140ced15d455e83941c5@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools