cmake: Do not clang-format check the externally imported json library

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@177795 91177308-0d34-0410-b5e6-96231b3b80d8
clang-format: Many more files

After this commit, polly is clang-format clean. This can be tested with
'ninja polly-check-format'. Updates to clang-format may change this, but the
differences will hopefully be both small and general improvements to the
formatting.

We currently have some not very nice formatting for a couple of items, DEBUG()
stmts for example. I believe the benefit of being clang-format clean outweights
the not perfect layout of this code.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@177796 91177308-0d34-0410-b5e6-96231b3b80d8