Introduce the notion of a "type validator" formatter

Type Validators have the purpose of looking at a ValueObject, and making sure that there is nothing semantically wrong about the object's contents
For instance, if you have a class that represents a speed, the validator might trigger if the speed value is greater than the speed of light

This first patch hooks up the moving parts in the formatters subsystem, but does not link ValueObjects to TypeValidators, nor lets the SB API be exposed to validators
It also lacks the notion of Python validators



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@217277 91177308-0d34-0410-b5e6-96231b3b80d8