cmd/compile: mark DIEs of captured variables

cmd/compile: mark DIEs of captured variables

Adds a new custom attribute to the DIE of captured variables,
containing the offset for the variable inside the closure struct. This
can be used by debuggers to display the contents of a closure variable.

Based on a sample program (delve) this increases the executable size by 0.06%.

benchstat output:

                         │   old.txt    │                new.txt                │
                         │    sec/op    │     sec/op      vs base               │
Template                   153.0m ±  6%    152.5m ±  14%       ~ (p=0.684 n=10)
Unicode                    100.2m ± 15%    104.9m ±   7%       ~ (p=0.247 n=10)
GoTypes                    943.6m ±  8%    986.2m ±  10%       ~ (p=0.280 n=10)
Compiler                   97.79m ±  6%   101.63m ±  12%       ~ (p=0.393 n=10)
SSA                         6.872 ± 37%     9.413 ± 106%       ~ (p=0.190 n=10)
Flate                      128.0m ± 36%    125.0m ±  56%       ~ (p=0.481 n=10)
GoParser                   214.9m ± 26%    201.4m ±  68%       ~ (p=0.579 n=10)
Reflect                    452.6m ± 22%    412.2m ±  74%       ~ (p=0.739 n=10)
Tar                        166.2m ± 27%    155.9m ±  73%       ~ (p=0.393 n=10)
XML                        219.3m ± 24%    211.3m ±  76%       ~ (p=0.739 n=10)
LinkCompiler               523.2m ± 13%    513.5m ±  47%       ~ (p=0.631 n=10)
ExternalLinkCompiler        1.684 ±  2%     1.659 ±  25%       ~ (p=0.218 n=10)
LinkWithoutDebugCompiler   304.9m ± 12%    309.1m ±   7%       ~ (p=0.631 n=10)
StdCmd                      70.76 ± 14%     68.66 ±  53%       ~ (p=1.000 n=10)
geomean                    511.5m          515.4m         +0.77%

                         │   old.txt    │               new.txt                │
                         │ user-sec/op  │  user-sec/op   vs base               │
Template                   269.6m ± 13%   292.3m ±  17%       ~ (p=0.393 n=10)
Unicode                    110.2m ±  8%   101.7m ±  18%       ~ (p=0.247 n=10)
GoTypes                     2.181 ±  9%    2.356 ±  12%       ~ (p=0.280 n=10)
Compiler                   119.1m ± 11%   121.9m ±  15%       ~ (p=0.481 n=10)
SSA                         17.75 ± 52%    26.94 ± 123%       ~ (p=0.190 n=10)
Flate                      256.2m ± 43%   226.8m ±  73%       ~ (p=0.739 n=10)
GoParser                   427.0m ± 24%   422.3m ±  72%       ~ (p=0.529 n=10)
Reflect                    990.5m ± 23%   905.5m ±  75%       ~ (p=0.912 n=10)
Tar                        307.9m ± 27%   308.9m ±  64%       ~ (p=0.393 n=10)
XML                        432.8m ± 24%   427.6m ±  89%       ~ (p=0.796 n=10)
LinkCompiler               796.9m ± 14%   800.4m ±  56%       ~ (p=0.481 n=10)
ExternalLinkCompiler        1.666 ±  4%    1.671 ±  28%       ~ (p=0.971 n=10)
LinkWithoutDebugCompiler   316.7m ± 12%   325.6m ±   8%       ~ (p=0.579 n=10)
geomean                    579.5m         594.0m         +2.51%

          │   old.txt    │                new.txt                │
          │  text-bytes  │  text-bytes   vs base                 │
HelloSize   842.9Ki ± 0%   842.9Ki ± 0%       ~ (p=1.000 n=10) ¹
CmdGoSize   10.95Mi ± 0%   10.95Mi ± 0%       ~ (p=1.000 n=10) ¹
geomean     3.003Mi        3.003Mi       +0.00%
¹ all samples are equal

          │   old.txt    │                new.txt                │
          │  data-bytes  │  data-bytes   vs base                 │
HelloSize   15.08Ki ± 0%   15.08Ki ± 0%       ~ (p=1.000 n=10) ¹
CmdGoSize   314.7Ki ± 0%   314.7Ki ± 0%       ~ (p=1.000 n=10) ¹
geomean     68.88Ki        68.88Ki       +0.00%
¹ all samples are equal

          │   old.txt    │                new.txt                │
          │  bss-bytes   │  bss-bytes    vs base                 │
HelloSize   396.8Ki ± 0%   396.8Ki ± 0%       ~ (p=1.000 n=10) ¹
CmdGoSize   428.8Ki ± 0%   428.8Ki ± 0%       ~ (p=1.000 n=10) ¹
geomean     412.5Ki        412.5Ki       +0.00%
¹ all samples are equal

          │   old.txt    │               new.txt               │
          │  exe-bytes   │  exe-bytes    vs base               │
HelloSize   1.310Mi ± 0%   1.310Mi ± 0%  +0.02% (p=0.000 n=10)
CmdGoSize   16.37Mi ± 0%   16.38Mi ± 0%  +0.01% (p=0.000 n=10)
geomean     4.631Mi        4.632Mi       +0.02%

Change-Id: Ib416ee2d916ec61ad4a5c26bab09597595f57e04
Reviewed-on: https://go-review.googlesource.com/c/go/+/563816
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>