Revise polly-{update|check}-format targets

Summary:
Make clang-format run on each file independently using
add_custom_format (instead using a shell script in utils/). The targets
polly-{update|check}-format depend on these.

The primary motivation is to make them work on Windows, but also
improves them generally:
- Each file update/check can run in parallel (Although they do not take
  long to run anyway)
- Implicit dependency on clang-format, so it recompiles if necessary
- polly-check-format shows the formatting difference if failing

Differential Revision: http://reviews.llvm.org/D12837

llvm-svn: 247581