Clarified the intent of the SymbolContextScope class in the header

documentation. Symbol now inherits from the symbol
context scope so that the StackID can use a "SymbolContextScope *"
instead of a blockID (which could have been the same as some other
blockID from another symbol file). 

Modified the stacks that are created on subsequent stops to reuse
the previous stack frame objects which will allow for some internal
optimization using pointer comparisons during stepping. 

llvm-svn: 112495