cmd/compile: directly construct Fields instead of ODCLFIELD nodes

cmd/compile: directly construct Fields instead of ODCLFIELD nodes

Avoids some garbage allocations while loading import data. Seems to
especially benefit html/template for some reason, but significant
allocation improvements for other packages too.

name       old time/op     new time/op     delta
Template       345ms ± 6%      332ms ± 6%   -3.76%        (p=0.000 n=49+47)
Unicode        185ms ±10%      184ms ±12%     ~           (p=0.401 n=50+49)
GoTypes        1.04s ± 3%      1.04s ± 3%   -0.72%        (p=0.012 n=48+47)
Compiler       4.52s ± 7%      4.49s ± 9%     ~           (p=0.465 n=48+47)

name       old user-ns/op  new user-ns/op  delta
Template        532M ±17%       471M ±23%  -11.48%        (p=0.000 n=50+50)
Unicode         298M ±29%       311M ±28%     ~           (p=0.065 n=50+50)
GoTypes        1.52G ± 7%      1.54G ± 9%     ~           (p=0.062 n=49+50)
Compiler       6.37G ± 7%      6.42G ± 8%     ~           (p=0.157 n=49+48)

name       old alloc/op    new alloc/op    delta
Template      43.9MB ± 0%     42.3MB ± 0%   -3.51%        (p=0.000 n=48+48)
Unicode       34.3MB ± 0%     34.3MB ± 0%     ~           (p=0.945 n=50+50)
GoTypes        123MB ± 0%      122MB ± 0%   -0.82%        (p=0.000 n=50+50)
Compiler       522MB ± 0%      519MB ± 0%   -0.51%        (p=0.000 n=50+50)

name       old allocs/op   new allocs/op   delta
Template        414k ± 0%       397k ± 0%   -4.14%        (p=0.000 n=50+49)
Unicode         320k ± 0%       320k ± 0%     ~           (p=0.988 n=48+49)
GoTypes        1.18M ± 0%      1.17M ± 0%   -0.97%        (p=0.000 n=50+50)
Compiler       4.44M ± 0%      4.41M ± 0%   -0.66%        (p=0.000 n=50+50)

Passes toolstash.

Change-Id: I0f54c0fa420d4f4ed3584c47cec0dde100c70c03
Reviewed-on: https://go-review.googlesource.com/31670
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>