Add a GetDisplayName() API to SBFrame, SBFunction and SBSymbol

This API is currently a no-op (in the sense that it has the same behavior as the already existing GetName()), but is meant long-term to provide a best-for-visualization version of the name of a function

It is still not hooked up to the command line 'bt' command, nor to the 'gui' mode, but I do have ideas on how to make that work going forward

rdar://21203242

llvm-svn: 241482