Emit 'Deprecated' Diagnostic messages

[Strum355]

The Go wiki outlines a standard for marking constructs as Deprecated. We should emit these as LSIF Diagnostics to support querying for usages of deprecated APIs.

Follow-on: what else can/should we emit? Are there existing tools we can utilize (cursed raw text output aside)? gopls/govet/staticcheck/golanglint-ci

[macraig]

Possibly handled by #141