The only new Types that the backend introduces
are pointers to Types generated by the frontend.
Usually, when we generate a *T,
we cache the resulting Type in T,
to avoid recreating it later.
However, that caching is not concurrency safe.
Rather than add mutexes, this CL disables that
caching before starting the backend.
The backend generates few enough new *Ts that the
performance impact of this is small, particularly
if we pre-create some commonly used *Ts.

Updates #15756

name       old alloc/op    new alloc/op    delta
Template      40.3MB ± 0%     40.4MB ± 0%  +0.18%  (p=0.001 n=10+10)
Unicode       29.8MB ± 0%     29.8MB ± 0%  +0.11%  (p=0.043 n=10+9)
GoTypes        114MB ± 0%      115MB ± 0%  +0.33%  (p=0.000 n=9+10)
SSA            855MB ± 0%      859MB ± 0%  +0.40%  (p=0.000 n=10+10)
Flate         25.7MB ± 0%     25.8MB ± 0%  +0.35%  (p=0.000 n=10+10)
GoParser      31.9MB ± 0%     32.1MB ± 0%  +0.58%  (p=0.000 n=10+10)
Reflect       79.6MB ± 0%     79.9MB ± 0%  +0.31%  (p=0.000 n=10+10)
Tar           26.9MB ± 0%     26.9MB ± 0%  +0.21%  (p=0.000 n=10+10)
XML           42.5MB ± 0%     42.7MB ± 0%  +0.52%  (p=0.000 n=10+9)

name       old allocs/op   new allocs/op   delta
Template        394k ± 1%       393k ± 0%    ~     (p=0.529 n=10+10)
Unicode         319k ± 1%       319k ± 0%    ~     (p=0.720 n=10+9)
GoTypes        1.15M ± 0%      1.15M ± 0%  +0.14%  (p=0.035 n=10+10)
SSA            7.53M ± 0%      7.56M ± 0%  +0.45%  (p=0.000 n=9+10)
Flate           238k ± 0%       238k ± 1%    ~     (p=0.579 n=10+10)
GoParser        318k ± 1%       320k ± 1%  +0.64%  (p=0.001 n=10+10)
Reflect        1.00M ± 0%      1.00M ± 0%    ~     (p=0.393 n=10+10)
Tar             254k ± 0%       254k ± 1%    ~     (p=0.075 n=10+10)
XML             395k ± 0%       397k ± 0%  +0.44%  (p=0.001 n=10+9)

Change-Id: I6c031ed4f39108f26969c5712b73aa2fc08cd10a
Reviewed-on: https://go-review.googlesource.com/38417
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>