Change Target::EvaluateExpression to take an ExecutionContextScope * rather than a

StackFrame * (StackFrame is an ExecutionContextScope.)  That allows you to call an
expression on a particular Thread, but not using the context of any particular frame.
That in turn is useful for injecting utility functions that don't actually depend on
locals/self/etc of the current frame.

I also had to include StackFrame.h in a couple of places so the compiler knew
how to downcast StackFrame to ExecutionContextScope.

<rdar://problem/22852953>

llvm-svn: 251564