Rename SignatureFlags enum and improve its usage
Rename SignatureFlags enum and improve its usage

As requested in the PR comments

microsoft/TypeScript,Eyas/TypeScript,microsoft/TypeScript,TukekeSoft/TypeScript,RyanCavanaugh/TypeScript,kpreisser/TypeScript,weswigham/TypeScript,RyanCavanaugh/TypeScript,SaschaNaz/TypeScript,SaschaNaz/TypeScript,alexeagle/TypeScript,alexeagle/TypeScript,nojvek/TypeScript,chuckjaz/TypeScript,nojvek/TypeScript,basarat/TypeScript,microsoft/TypeScript,minestarks/TypeScript,basarat/TypeScript,Microsoft/TypeScript,nojvek/TypeScript,synaptek/TypeScript,kpreisser/TypeScript,nojvek/TypeScript,donaldpipowitch/TypeScript,SaschaNaz/TypeScript,chuckjaz/TypeScript,kpreisser/TypeScript,TukekeSoft/TypeScript,synaptek/TypeScript,TukekeSoft/TypeScript,RyanCavanaugh/TypeScript,donaldpipowitch/TypeScript,chuckjaz/TypeScript,minestarks/TypeScript,SaschaNaz/TypeScript,donaldpipowitch/TypeScript,basarat/TypeScript,Eyas/TypeScript,kitsonk/TypeScript,basarat/TypeScript,Microsoft/TypeScript,Eyas/TypeScript,weswigham/TypeScript,kitsonk/TypeScript,synaptek/TypeScript,Microsoft/TypeScript,kitsonk/TypeScript,alexeagle/TypeScript,minestarks/TypeScript,weswigham/TypeScript,donaldpipowitch/TypeScript,synaptek/TypeScript,Eyas/TypeScript,chuckjaz/TypeScript