go/doc: add IsPredeclared function
go/doc: add IsPredeclared function

IsPredeclared allows simplifying src/golang.org/x/tools/godoc/linkify.go

Change-Id: I56b3223896f844630bc2e940255572d1682f0d06
Reviewed-on: https://go-review.googlesource.com/29870
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

nawawi/go,ImJasonH/go,akutz/go,Cofyc/go,akutz/go,golang/go,momchil-velikov/go,akutz/go,goodlang/good,ImJasonH/go,Cofyc/go,suncycheng/go,momchil-velikov/go,CAFxX/go,CAFxX/go,momchil-velikov/go,Cofyc/go,linux-on-ibm-z/go,cloudflare/go,akutz/go,nawawi/go,josharian/go,cloudflare/go,cloudflare/go,ImJasonH/go,Cofyc/go,codestation/go,momchil-velikov/go,codestation/go,CAFxX/go,josharian/go,polyverse-security/go,vsekhar/elastic-go,derekmarcotte/go,codestation/go,josharian/go,derekmarcotte/go,suncycheng/go,cloudflare/go,derekmarcotte/go,CAFxX/go,derekmarcotte/go,goodlang/good,polyverse-security/go,ImJasonH/go,golang/go,CAFxX/go,vsekhar/elastic-go,polyverse-security/go,linux-on-ibm-z/go,suncycheng/go,nawawi/go,Cofyc/go,goodlang/good,akutz/go,golang/go,Cofyc/go,derekmarcotte/go,vsekhar/elastic-go,vsekhar/elastic-go,nawawi/go,cloudflare/go,linux-on-ibm-z/go,linux-on-ibm-z/go,CAFxX/go,polyverse-security/go,momchil-velikov/go,golang/go,suncycheng/go,Cofyc/go,nawawi/go,vsekhar/elastic-go,polyverse-security/go,linux-on-ibm-z/go,codestation/go,ImJasonH/go,CAFxX/go,golang/go,akutz/go,linux-on-ibm-z/go,goodlang/good,christopher-henderson/Go,josharian/go,cloudflare/go,goodlang/good,suncycheng/go,ImJasonH/go,polyverse-security/go,nawawi/go,linux-on-ibm-z/go,linux-on-ibm-z/go,momchil-velikov/go,cloudflare/go,vsekhar/elastic-go,josharian/go,suncycheng/go,derekmarcotte/go,josharian/go,goodlang/good,polyverse-security/go,ImJasonH/go,vsekhar/elastic-go,derekmarcotte/go,akutz/go,josharian/go,codestation/go,codestation/go,golang/go,codestation/go,ImJasonH/go,nawawi/go,Cofyc/go,golang/go,suncycheng/go,codestation/go,polyverse-security/go,golang/go,derekmarcotte/go,momchil-velikov/go,CAFxX/go,momchil-velikov/go,akutz/go,suncycheng/go,vsekhar/elastic-go,goodlang/good,josharian/go,cloudflare/go,goodlang/good,nawawi/go