ast: make ExampleCommentMap a runnable example
ast: make ExampleCommentMap a runnable example

Fixes #20450

Change-Id: I2256282a8880e99508e98fefedfb94a7cccacbcf
Reviewed-on: https://go-review.googlesource.com/48969
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

Cofyc/go,polyverse-security/go,nawawi/go,CAFxX/go,vsekhar/elastic-go,josharian/go,CAFxX/go,cloudflare/go,golang/go,christopher-henderson/Go,cloudflare/go,Cofyc/go,codestation/go,golang/go,nawawi/go,Cofyc/go,nawawi/go,nawawi/go,Cofyc/go,polyverse-security/go,akutz/go,akutz/go,codestation/go,nawawi/go,Cofyc/go,golang/go,codestation/go,josharian/go,Cofyc/go,cloudflare/go,josharian/go,polyverse-security/go,vsekhar/elastic-go,josharian/go,josharian/go,Cofyc/go,vsekhar/elastic-go,CAFxX/go,josharian/go,nawawi/go,codestation/go,josharian/go,Cofyc/go,vsekhar/elastic-go,nawawi/go,akutz/go,codestation/go,akutz/go,cloudflare/go,polyverse-security/go,vsekhar/elastic-go,akutz/go,akutz/go,polyverse-security/go,codestation/go,polyverse-security/go,CAFxX/go,cloudflare/go,akutz/go,codestation/go,CAFxX/go,polyverse-security/go,CAFxX/go,codestation/go,cloudflare/go,golang/go,cloudflare/go,golang/go,CAFxX/go,josharian/go,golang/go,vsekhar/elastic-go,polyverse-security/go,vsekhar/elastic-go,CAFxX/go,cloudflare/go,akutz/go,nawawi/go,golang/go,vsekhar/elastic-go,golang/go