flag: add BoolFunc; FlagSet.BoolFunc
Fixes #53747
Based on CL 416514
Change-Id: I1ff79c6290b06dfa8672a473045e8fe80c22afcf
GitHub-Last-Rev:
74fba9b3096487c04c8dc1f2237f67f3558212f1
GitHub-Pull-Request: golang/go#59013
Reviewed-on: https://go-review.googlesource.com/c/go/+/476015
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>