clang-format: use AfterControlStatement to format ObjC control blocks

ObjC defines `@autoreleasepool` and `@synchronized` control blocks. These
used to be formatted according to the `AfterObjCDeclaration` brace-
wrapping flag, which is not very consistent.

This patch changes the behavior to use the `AfterControlStatement` flag
instead. This should not affect the behavior unless a custom brace
wrapping mode is used.

Reviewers: krasimir, djasper, klimek, benhamilton

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D43232

llvm-svn: 326192