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'd like to use Type.cmp for sorting,
but that causes infinite recursion at the moment;
see #19869.

For now, use Type.LongString as the sort key,
which is a complete description of the type.
Type.LongString is relatively expensive,
but we calculate it only once per type,
and signatlist is generally fairly small,
so the performance impact is minimal.

Updates #15756

name       old alloc/op    new alloc/op    delta
Template      39.4MB ± 0%     39.4MB ± 0%    ~     (p=0.222 n=5+5)
Unicode       29.8MB ± 0%     29.8MB ± 0%    ~     (p=0.151 n=5+5)
GoTypes        113MB ± 0%      113MB ± 0%    ~     (p=0.095 n=5+5)
SSA           1.25GB ± 0%     1.25GB ± 0%  +0.04%  (p=0.008 n=5+5)
Flate         25.3MB ± 0%     25.4MB ± 0%    ~     (p=0.056 n=5+5)
GoParser      31.8MB ± 0%     31.8MB ± 0%    ~     (p=0.310 n=5+5)
Reflect       78.3MB ± 0%     78.3MB ± 0%    ~     (p=0.690 n=5+5)
Tar           26.7MB ± 0%     26.7MB ± 0%    ~     (p=0.548 n=5+5)
XML           42.2MB ± 0%     42.2MB ± 0%    ~     (p=0.222 n=5+5)

name       old allocs/op   new allocs/op   delta
Template        387k ± 0%       388k ± 0%    ~     (p=0.056 n=5+5)
Unicode         320k ± 0%       321k ± 0%  +0.32%  (p=0.032 n=5+5)
GoTypes        1.14M ± 0%      1.15M ± 0%    ~     (p=0.095 n=5+5)
SSA            9.70M ± 0%      9.72M ± 0%  +0.18%  (p=0.008 n=5+5)
Flate           234k ± 0%       235k ± 0%  +0.60%  (p=0.008 n=5+5)
GoParser        317k ± 0%       317k ± 0%    ~     (p=1.000 n=5+5)
Reflect         982k ± 0%       983k ± 0%    ~     (p=0.841 n=5+5)
Tar             252k ± 1%       252k ± 0%    ~     (p=0.310 n=5+5)
XML             393k ± 0%       392k ± 0%    ~     (p=0.548 n=5+5)

Change-Id: I53a3b95d19cf1a7b7511a94fba896706addf84fb
Reviewed-on: https://go-review.googlesource.com/39710
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>