go/doc: add new mode bit PreserveAST to control clearing of data in AST
To save memory in godoc, this package routinely clears fields of
the AST to avoid keeping data that godoc no longer needs. For other
programs, such as cmd/doc, this behavior is unfortunate. Also, one
should be able to tell any package like this, "don't change my
data".
Add a Mode bit, defaulting to off to preserve existing behavior,
that allows a client to specify that the AST is inviolate.
This is necessary to address some of the outstanding issues
in cmd/doc that require, for example, looking at function bodies.
Fixes #26835
Change-Id: I01cc97c6addc5ab6abff885fff4bd53454a03bbc
Reviewed-on: https://go-review.googlesource.com/c/140958 Reviewed-by: Robert Griesemer <gri@golang.org>