gopls: support assembly References for labels and asm-only symbols

[groot-guo]

Extend textDocument/references in Go assembly files to support control labels, file-local symbols, and symbols without Go declarations.
Control labels are scoped to the enclosing TEXT function, and file-local <> symbols are scoped to their source file.

For golang/go#71754

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/809420.
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[gopherbot]

Message from 郭根源（genyuan）:

Patch Set 2:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/809420.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

This PR (HEAD: 50917c7) has been imported to Gerrit for code review.

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Important tips:

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