cmake: add command polly-check-format

This command runs clang-format on all files and reports formatting problems

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@175170 91177308-0d34-0410-b5e6-96231b3b80d8
Report formatting problems as with 'diff -u'

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@175171 91177308-0d34-0410-b5e6-96231b3b80d8
clang-format: Really check all files

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@175174 91177308-0d34-0410-b5e6-96231b3b80d8
Formatting fixes

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@175177 91177308-0d34-0410-b5e6-96231b3b80d8
Do not run formatting checks by default

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@175178 91177308-0d34-0410-b5e6-96231b3b80d8