gopls/internal/lsp: add go to implementation support for function types

[a-h]

Fixes #56572

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Message from Robert Findley:

Patch Set 1:

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Message from Alan Donovan:

Patch Set 1:

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Patch Set 2:

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Patch Set 2:

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Patch Set 4:

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[a-h]

Hey @gopherbot - how come you haven't picked up the latest commit yet?

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Patch Set 5:

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Patch Set 5:

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Patch Set 5:

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Patch Set 5:

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[IceflowRE]

Does it work with generics?

[a-h]

I took a break from it to try and understand the Go language spec better, because I felt that I didn't fully understand all the points coming back the code review, and now it's out of date. I did think about getting back to this at some point.

There's no generic test case, so this PR might not.