cmd/internal/dwarf: fix missing decl_file tag for abstract subprograms

[piob-io]

Abstract subprograms are declared at specific file:line just as regular
subprograms. Line annotation was already added, but as todo notes,
not the file.

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Patch Set 1:

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[gopherbot]

Message from Piotr Bejda:

Patch Set 1:

(1 comment)

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