Move `isConstantString()` to base class
Move `isConstantString()` to base class

Summary: This can be useful by itself, we want to use it in FB linter.

Test Plan: This diff.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4744

fivestars/arcanist,codelabs-ch/arcanist,wikimedia/phabricator-arcanist,dguo-coursera/arcanist,codelabs-ch/arcanist,calblueprint/arcanist,hach-que/arcanist,fivestars/arcanist,Khan/arcanist,coursera/arcanist,aik099/arcanist,memsql/arcanist,wxstars/arcanist,jra3/arcanist,alexmv/arcanist,hach-que/arcanist,r4nt/arcanist,memsql/arcanist,haskell-infra/arcanist,haskell-infra/arcanist,wikimedia/phabricator-arcanist,kevinseelbach/arcanist,eSpark/arcanist,uber/arcanist,folsom-labs/arcanist,wharris/arcanist,Symplicity/arcanist,kevinseelbach/arcanist,wxstars/arcanist,memsql/arcanist,aik099/arcanist,eSpark/arcanist,hach-que/arcanist,calblueprint/arcanist,Khan/arcanist,sneakpeeq/arcanist,julkiewicz/arcanist,jra3/arcanist,eSpark/arcanist,Khan/arcanist,PageUpPeopleOrg/arcanist-patches,jra3/arcanist,phacility/arcanist,wxstars/arcanist,phacility/arcanist,chundongwang/arcanist,folsom-labs/arcanist,haskell-infra/arcanist,KieranHayes/arcanist,zhihu/arcanist,julkiewicz/arcanist,dguo-coursera/arcanist,alexmv/arcanist,wharris/arcanist,kevinseelbach/arcanist,KieranHayes/arcanist,uber/arcanist,alexmv/arcanist,zhihu/arcanist,codelabs-ch/arcanist,sneakpeeq/arcanist,aik099/arcanist,wxstars/arcanist,coursera/arcanist,memsql/arcanist,r4nt/arcanist,fivestars/arcanist,kevinseelbach/arcanist,zhihu/arcanist,alexmv/arcanist,aik099/arcanist,chundongwang/arcanist,wharris/arcanist,folsom-labs/arcanist,KieranHayes/arcanist,dguo-coursera/arcanist,haskell-infra/arcanist,Symplicity/arcanist,julkiewicz/arcanist,folsom-labs/arcanist,coursera/arcanist,chundongwang/arcanist,sneakpeeq/arcanist,uber/arcanist,codelabs-ch/arcanist,r4nt/arcanist,PageUpPeopleOrg/arcanist-patches,dguo-coursera/arcanist,eSpark/arcanist,wharris/arcanist,PageUpPeopleOrg/arcanist-patches,phacility/arcanist,julkiewicz/arcanist,coursera/arcanist,calblueprint/arcanist,hach-que/arcanist,zhihu/arcanist,fivestars/arcanist,wikimedia/phabricator-arcanist,jra3/arcanist,wikimedia/phabricator-arcanist,Khan/arcanist,sneakpeeq/arcanist,Symplicity/arcanist,phacility/arcanist,r4nt/arcanist,KieranHayes/arcanist,phacility/arcanist,PageUpPeopleOrg/arcanist-patches,Symplicity/arcanist,uber/arcanist,calblueprint/arcanist,chundongwang/arcanist,codelabs-ch/arcanist