Simplify 'Environment' to contain only one map from 'const Stmt*' to SVals,...

Simplify 'Environment' to contain only one map from 'const Stmt*' to SVals, greatly simplifying the logic of the analyzer in many places.  We now only distinguish between block-level expressions and subexpressions in Environment::RemoveDeadBindings and GRState pretty-printing.

llvm-svn: 80194