Moving ValueObjectCast over to its own .h/.cpp files instead of sharing ValueObjectDynamic.h/.cpp

Removing the IsDynamic() and GetStaticValue() calls, so that they will default to the base class behavior:
 - non-dynamic
 - itself as the static value
This is in contrast with the previous behavior which could be confusing and could potentially cause issues when using those objects

llvm-svn: 166857