x/tools/gopls: preserve comments when invoking fillstruct on partially filled composite literals

[findleyr]

It looks like in #39804, there was a desire to preserve comments when filling partial literals, but it was deemed too challenging.

I think with recent work by @madelinekalil to reassemble the resulting literal, this should be a solvable problem, and a nice UX improvement.

Tentatively assigning for v0.18.0. It would be nice to bundle this improvement.

[xzbdmw]

I think with recent work by @madelinekalil to reassemble the resulting literal, this should be a solvable problem, and a nice UX improvement.

Does this mean we manipulate string literals for each field? I was trying to attach snippets on fillstrcut when clients have capabilities (#70432) , and thought I have to do that too.