[analyzer] When binding to a ParenExpr, bind to its inner expression instead.

This actually looks through several kinds of expression, such as
OpaqueValueExpr and ExprWithCleanups. The idea is that binding and lookup
should be consistent, and so if the environment needs to be modified later,
the code doing the modification will not have to manually look through these
"transparent" expressions to find the real binding to change.

This is necessary for proper updating of struct rvalues as described in
the previous commit.

llvm-svn: 166121