[dev.regabi] cmd/compile: simplify function/interface/struct typechecking

After the previous CL, the only callers to NewFuncType, tointerface,
or NewStructType are the functions for type-checking the type literal
ASTs. So just inline the code there.

While here, refactor the Field type-checking logic a little bit, to
reduce some duplication.

Passes toolstash -cmp.

Change-Id: Ie12d14b87ef8b6e528ac9dccd609604bd09b98ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/279956


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>