StringRefize TargetInfo::getABI()
Avoid dubious IdentifierInfo::getNameStart() uses

These cases in particular were incurring an extra strlen() when we already knew
the length. They appear to be leftovers from when the interfaces worked with C
strings that have continued to compile due to the implicit StringRef ctor.
DeclObjC: Fix comments about bad API

What's going on here isn't overriding but rather the sketchy practice of member
hiding (shadowing of base members).