reflect: optimize (reflect.Type).Name

reflect: optimize (reflect.Type).Name

Improves JSON decoding on linux/amd64.

name                   old time/op    new time/op    delta
CodeUnmarshal-40         89.3ms ± 2%    86.3ms ± 2%  -3.31%  (p=0.000 n=22+22)

name                   old speed      new speed      delta
CodeUnmarshal-40       21.7MB/s ± 2%  22.5MB/s ± 2%  +3.44%  (p=0.000 n=22+22)

Updates #16117

Change-Id: I52acf31d7729400cfe6693e46292d41e1addba3d
Reviewed-on: https://go-review.googlesource.com/24410
Run-TryBot: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>