Introduce the notion of "runtime support values"

A runtime support value is a ValueObject whose only purpose is to support some language runtime's operation, but it does not directly provide any user-visible benefit
As such, unless the user is working on the runtime support, it is mostly safe for them not to see such a value when debugging

It is a language runtime's job to check whether a ValueObject is a support value, and that - in conjunction with a target setting - is used by frame variable and target variable
SBFrame::GetVariables gets a new overload with yet another flag to dictate whether to return those support values to the caller - that which defaults to the setting's value

rdar://problem/15539930

llvm-svn: 228791