[analyzer] Remove the "direct bindings only" Environment lookup.

This was only used by OSAtomicChecker and makes it more
difficult to update values for expressions that the environment
may look through instead (it's not the same as IgnoreParens).
With this gone, we can have bindExpr bind to the inner
expression that getSVal will find.

Groundwork for <rdar://problem/12137950>

llvm-svn: 165866