[release-branch.go1.12] cmd/cgo: simplify and fix handling of untyped constants

Instead of trying to guess type of constants in the AST,
which is hard, use the "var cgo%d Type = Constant"
so that typechecking is left to the Go compiler.

The previous code could still fail in some cases
for constants imported from other modules
or defined in other, non-cgo files.

Fixes #30527

Change-Id: I2120cd90e90a74b9d765eeec53f6a3d2cfc1b642
Reviewed-on: https://go-review.googlesource.com/c/go/+/164897


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit 711ea1e7)
Reviewed-on: https://go-review.googlesource.com/c/go/+/165748


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>