This is a re-roll of CL 39710,
which broke deterministic builds.

typenamesym is called from three places:
typename, ngotype, and Type.Symbol.
Only in typename do we actually need a Node.
ngotype and Type.Symbol require only a Sym.
And writing the newly created Node to
Sym.Def is unsafe in a concurrent backend.
Rather than use a mutex protect to Sym.Def,
make typenamesym not touch Sym.Def.

The assignment to Sym.Def was serving a second purpose,
namely to prevent duplicate entries on signatlist.
Preserve that functionality by switching signatlist to a map.
This in turn requires that we sort signatlist
when exporting it, to preserve reproducibility.

We sort using exactly the same mechanism
that the export code (dtypesym) uses.
Failure to do that led to non-deterministic builds (#19872).
Since we've already calculated the Type's export name,
we could pass it to dtypesym, sparing it a bit of work.
That can be done as a future optimization.

Updates #15756

name       old alloc/op      new alloc/op      delta
Template        39.2MB ± 0%       39.3MB ± 0%    ~     (p=0.075 n=10+10)
Unicode         29.8MB ± 0%       29.8MB ± 0%    ~     (p=0.393 n=10+10)
GoTypes          113MB ± 0%        113MB ± 0%  +0.06%  (p=0.027 n=10+8)
SSA             1.25GB ± 0%       1.25GB ± 0%  +0.05%  (p=0.000 n=8+10)
Flate           25.3MB ± 0%       25.3MB ± 0%    ~     (p=0.105 n=10+10)
GoParser        31.7MB ± 0%       31.8MB ± 0%    ~     (p=0.165 n=10+10)
Reflect         78.2MB ± 0%       78.2MB ± 0%    ~     (p=0.190 n=10+10)
Tar             26.6MB ± 0%       26.6MB ± 0%    ~     (p=0.481 n=10+10)
XML             42.2MB ± 0%       42.2MB ± 0%    ~     (p=0.968 n=10+9)

name       old allocs/op     new allocs/op     delta
Template          384k ± 1%         386k ± 1%  +0.43%  (p=0.019 n=10+10)
Unicode           320k ± 0%         321k ± 0%  +0.36%  (p=0.015 n=10+10)
GoTypes          1.14M ± 0%        1.14M ± 0%  +0.33%  (p=0.000 n=10+8)
SSA              9.69M ± 0%        9.71M ± 0%  +0.18%  (p=0.000 n=10+9)
Flate             233k ± 1%         233k ± 1%    ~     (p=0.481 n=10+10)
GoParser          315k ± 1%         316k ± 1%    ~     (p=0.113 n=9+10)
Reflect           979k ± 0%         979k ± 0%    ~     (p=0.971 n=10+10)
Tar               250k ± 1%         250k ± 1%    ~     (p=0.481 n=10+10)
XML               391k ± 1%         392k ± 0%    ~     (p=1.000 n=10+9)

Change-Id: Ia9f21cc29c047021fa8a18c2a3d861a5146aefac
Reviewed-on: https://go-review.googlesource.com/39915
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>