go/types: add UsesCgo config to support _cgo_gotypes.go
This CL adds a UsesCgo config setting to go/types to specify that the
_cgo_gotypes.go file generated by cmd/cgo has been provided as a
source file. The type checker then internally resolves C.bar qualified
identifiers to _Cfoo_bar as appropriate.
It also adds support to srcimporter to automatically run cgo.
Unfortunately, this functionality is not compatible with overriding
OpenFile, because cmd/cgo and gcc will directly open files.
Updates #16623.
Updates #35721.
Change-Id: I1e1965fe41b765b7a9da3431f2a86cc16025dee2
Reviewed-on: https://go-review.googlesource.com/c/go/+/33677
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>