clang-format: Teach --sort-includes to interleave #include and #import.

clang accepts both #include and #import for includes (the latter having an
implicit header guard).  Let clang-format interleave both types if
--sort-includes is passed.  #import is used frequently in Objective-C code.

http://reviews.llvm.org/D13853

llvm-svn: 250909