lsp/source: create zero value ast.Exprs
lsp/source: create zero value ast.Exprs

In support of https://golang.org/cl/224960, add utility functions that
create zero value expressions for a given types.Type. There are probably
some bugs here but they pass the basic tests in that CL.

Known problems: it skips through type aliases, and can't handle
anonymous structs/interfaces. Hopefully it's pretty unusual to return
those.

Change-Id: I69a388d9ce9bd60bc230e4a3d2b30f581ec25698
Reviewed-on: https://go-review.googlesource.com/c/tools/+/225177
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Rohan Challa <9671c73253cee9b18b8df367e3e627fab2863fe8@golang.org>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

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