Handle the case when a variable is only valid in part of the enclosing scope

DWARF stores this information in the DW_AT_start_scope attribute. This
CL add support for this attribute and also changes the functions
displaying frame variables to only display the variables currently in
scope.

Differential revision: http://reviews.llvm.org/D17449

Author:    Tamas Berghammer <tberghammer@google.com>
Date:      Thu Feb 25 12:23:37 2016 +0000

This is a cherry-pick of r261858

llvm-svn: 270119