CL 38147 eliminated package gc globals in formatting routines.
However, tconv still used the Type field Trecur
to avoid infinite recursion when formatting recursive
interfaces types such as (test/fixedbugs398.go):

type i1 interface {
	F() interface {
		i1
	}
}

type i2 interface {
	F() interface {
		i2
	}
}

This CL changes the recursion prevention to use a parameter,
and threads it through the formatting routines.
Because this fundamentally limits the embedding depth
of all types, it sets the depth limit to be much higher.
In practice, it is unlikely to impact any code at all,
one way or the other.

The remaining uses of Type.Trecur are boolean in nature.
A future CL will change Type.Trecur to be a boolean flag.

The removal of a couple of mode.Sprintf calls
makes this a very minor net performance improvement:

name       old alloc/op    new alloc/op    delta
Template      40.0MB ± 0%     40.0MB ± 0%  -0.13%  (p=0.032 n=5+5)
Unicode       30.0MB ± 0%     29.9MB ± 0%    ~     (p=0.310 n=5+5)
GoTypes        114MB ± 0%      113MB ± 0%  -0.25%  (p=0.008 n=5+5)
SSA            856MB ± 0%      855MB ± 0%  -0.04%  (p=0.008 n=5+5)
Flate         25.5MB ± 0%     25.4MB ± 0%  -0.27%  (p=0.008 n=5+5)
GoParser      31.9MB ± 0%     31.9MB ± 0%    ~     (p=0.222 n=5+5)
Reflect       79.0MB ± 0%     78.6MB ± 0%  -0.45%  (p=0.008 n=5+5)
Tar           26.8MB ± 0%     26.7MB ± 0%  -0.25%  (p=0.032 n=5+5)
XML           42.4MB ± 0%     42.4MB ± 0%    ~     (p=0.151 n=5+5)

name       old allocs/op   new allocs/op   delta
Template        395k ± 0%       391k ± 0%  -1.00%  (p=0.008 n=5+5)
Unicode         321k ± 1%       319k ± 0%  -0.56%  (p=0.008 n=5+5)
GoTypes        1.16M ± 0%      1.14M ± 0%  -1.61%  (p=0.008 n=5+5)
SSA            7.63M ± 0%      7.60M ± 0%  -0.30%  (p=0.008 n=5+5)
Flate           239k ± 0%       234k ± 0%  -1.94%  (p=0.008 n=5+5)
GoParser        320k ± 0%       317k ± 1%  -0.86%  (p=0.008 n=5+5)
Reflect        1.00M ± 0%      0.98M ± 0%  -2.17%  (p=0.016 n=4+5)
Tar             255k ± 1%       251k ± 0%  -1.35%  (p=0.008 n=5+5)
XML             398k ± 0%       395k ± 0%  -0.89%  (p=0.008 n=5+5)

Updates #15756

Change-Id: Id23e647d347aa841f9a69d51f7d2d7d27b259239
Reviewed-on: https://go-review.googlesource.com/38797
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>