For struct fields and methods, Field.Nname was only used to store
position information, which means we're allocating an entire ONAME
Node+Name+Param structure just for one field. We can optimize away
these ONAME allocations by instead adding a Field.Pos field.

Unfortunately, we can't get rid of Field.Nname, because it's needed
for function parameters, so Field grows a little bit and now has more
redundant information in those cases. However, that was already the
case (e.g., Field.Sym and Field.Nname.Sym), and it's still a net win
for allocations as demonstrated by the benchmarks below.

Additionally, by moving the ONAME allocation for function parameters
to funcargs, we can avoid allocating them for function parameters that
aren't used in corresponding function bodies (e.g., interface methods,
function-typed variables, and imported functions/methods without
inline bodies).

name       old time/op       new time/op       delta
Template         254ms ± 6%        251ms ± 6%  -1.04%  (p=0.000 n=487+488)
Unicode          128ms ± 7%        128ms ± 7%    ~     (p=0.294 n=482+467)
GoTypes          862ms ± 5%        860ms ± 4%    ~     (p=0.075 n=488+471)
Compiler         3.91s ± 4%        3.90s ± 4%  -0.39%  (p=0.000 n=468+473)

name       old user-time/op  new user-time/op  delta
Template         339ms ±14%        336ms ±14%  -1.02%  (p=0.001 n=498+494)
Unicode          176ms ±18%        176ms ±25%    ~     (p=0.940 n=491+499)
GoTypes          1.13s ± 8%        1.13s ± 9%    ~     (p=0.157 n=496+493)
Compiler         5.24s ± 6%        5.21s ± 6%  -0.57%  (p=0.000 n=485+489)

name       old alloc/op      new alloc/op      delta
Template        38.3MB ± 0%       37.3MB ± 0%  -2.58%  (p=0.000 n=499+497)
Unicode         29.1MB ± 0%       29.1MB ± 0%  -0.03%  (p=0.000 n=500+493)
GoTypes          116MB ± 0%        115MB ± 0%  -0.65%  (p=0.000 n=498+499)
Compiler         492MB ± 0%        487MB ± 0%  -1.00%  (p=0.000 n=497+498)

name       old allocs/op     new allocs/op     delta
Template          364k ± 0%         360k ± 0%  -1.15%  (p=0.000 n=499+499)
Unicode           336k ± 0%         336k ± 0%  -0.01%  (p=0.000 n=500+493)
GoTypes          1.16M ± 0%        1.16M ± 0%  -0.30%  (p=0.000 n=499+499)
Compiler         4.54M ± 0%        4.51M ± 0%  -0.58%  (p=0.000 n=494+495)

Passes toolstash-check -gcflags=-dwarf=false. Changes DWARF output
because position information is now tracked more precisely for
function parameters.

Change-Id: Ib8077d70d564cc448c5e4290baceab3a4396d712
Reviewed-on: https://go-review.googlesource.com/108217
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>