[clang-format] square parens with one token are not Objective-C message sends

The commit r322690 introduced support for ObjC detection in header files.
Unfortunately some C headers that use designated initializers are now
incorrectly detected as Objective-C.
This commit fixes it by ensuring that `[ token ]` is not annotated as an
Objective-C message send.

rdar://45504376

Differential Revision: https://reviews.llvm.org/D56226

llvm-svn: 352125