Remove hack from LiveVariables analysis where variables whose address are taken

are considered 'live'. This hack isn't needed anymore because we have a
separation in the path-sensitive analyzer between variable names and bindings;
the analyzer can continue to reason about the storage of a variable after its
name is no longer directly referenced.  Now the live variables analysis literally means "is this name live".

Along this line, update the dead stores checker to explicitly look for variables
whose values have escaped.

llvm-svn: 68504