Add SignatureHelp functionality to source package. Tests will be added
in a subsequent change.
Change-Id: Ia43280946d96a984c5741273a00c12255d637b2a
Reviewed-on: https://go-review.googlesource.com/c/149177
Reviewed-by: Ian Cottrell <iancottrell@google.com>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
