[polly][NFC] removes using-directives to fix modules build

When compiling with Clang modules enabled, polly's use of using-directives
caused the global object `Target` in RegisterPasses.cpp to clash with
`llvm::Target`. By eliminating the using-directives, we're able to get
polly to play nicely with a modules build.

Differential Revision: https://reviews.llvm.org/D119809