Centralize the code for GetValueAsCString() in TypeFormatImpl (the...

Centralize the code for GetValueAsCString() in TypeFormatImpl (the implementing class of "type format ...")
TypeFormatImpl used to just wrap a Format (and Flags for matching), and then ValueObject itself would do the printing deed
With this checkin, the responsibility of generating a value string is centralized in the data formatter (as it should, and already is for summaries) 

This change is good practice per se, and should also enable us to extend the type format mechanism in a cleaner way

llvm-svn: 197874