go/types, types2: swap object.color for Checker.objPathIdx

The type checker assigns types to objects. An object can be in
1 of 3 states, which are mapped to colors. This is stored as a
field on the object.

 - white : type not known, awaiting processing
 - grey  : type pending, in processing
 - black : type known, done processing

With the addition of Checker.objPathIdx, which maps objects to
a path index, presence in the map could signal grey coloring.
White and black coloring can be signaled by presence of
object.typ (a simple nil check).

This change removes the object.color field and its associated
methods, replacing it with an equivalent use of Checker.objPathIdx.
Checker.objPathIdx is integrated into the existing push and pop
methods, while slightly simplifying their signatures.

Note that the concept of object coloring remains the same - we
are merely changing and simplifying the mechanism which represents
the colors.

Change-Id: I91fb5e9a59dcb34c08ffc5b4ebc3f20a400094b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/715840
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Mark Freeman <markfreeman@google.com>

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