Added access to set the current stack frame within a thread so any command

line commands can use the current thread/frame.

Fixed an issue with expressions that get sandboxed in an objective C method
where unichar wasn't being passed down.

Added a "static size_t Scalar::GetMaxByteSize();" function in case we need
to know the max supported by size of something within a Scalar object.

llvm-svn: 122027