Add .clang-format to parallel-libs

Summary:
The format style is set to LLVM. This is consistent with the
parallel-libs project charter which specifies that its libraries will
conform to LLVM coding style.

Reviewers: jlebar

Subscribers: parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D22576

llvm-svn: 276145
GitOrigin-RevId: a12aa1fa9ce17bbf99d440f8f2e856b1758f9a15