This commit removes `sw::Value`, `sw::BasicBlock`, and
`sw::SwitchCases`.  In LLVM 7.0, `llvm::Value`, `llvm::BasicBlock`, and
`llvm::SwitchCases` become final classes, thus we cannot derive from
those classes.  Fortunately, we don't have to derive from them.  We can
explicitly cast the pointers instead.

Change-Id: I2dc74485ebf4b399f8a41bb8fa94b7d8d299009e
Reviewed-on: https://swiftshader-review.googlesource.com/20469Tested-by: Logan Chien <loganchien@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>