internal/lsp: revert to old method of computing error ranges
internal/lsp: revert to old method of computing error ranges

The approach of using ASTs to determine error ranges had more
complications than I anticipated. Revert it for now to go back to the
old user experience, while we consider a better approach.

Change-Id: I5b23f0147c26089330f8a4bbf7b6914ae66cf59a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/204561
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools