internal/lsp/cache: fix type error reporting in cgo
internal/lsp/cache: fix type error reporting in cgo

When dealing with cgo files, we need to pass a Mapper for the authored
file. I got most of the sites in CL 208501, but missed this one.

I was nervous about starting to propagate errors, but all the tests
passed, so shrug.

Change-Id: I7505e670e9c01d719c72c3f99d01c8153c3d2ff5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/212862
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

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