go/parser: switch to resolving objects as a post-processing pass

Coupling object resolution to parsing complicates the parsing code, and
is a barrier to improvement. It requires passing around context such as
'lhs' or 'keyOk', and even then sometimes requires guess-work, such as
whether to resolve the key in a composite literal.

In this CL we delay object resolution to a separate pass after the file
parse completes. This makes it easier to see logic of scoping, and
removes state from the parsing code. This can enable subsequent
improvements such as optionally skipping object resolution, aligning the
parser with cmd/compile/internal/syntax, and allowing alternative
parsers to reuse object resolution.

The additional AST traversal appears to slow down parsing by around 4%.
That seems small enough not to worry about, especially since performance
sensitive users may eventually be able to disable object resolution
entirely, saving around 18% off the previous baseline. I'll also mail a
speculative CL showing how we can significantly mitigate the cost of
object resolution by transposing scopes.

For #45104

Change-Id: I98d9143fd77ae29e84ec7c3ae2fdb1139510da37
Reviewed-on: https://go-review.googlesource.com/c/go/+/304455


Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>