clang-format: fix formatting of ObjC @synchronized blocks

Summary:
The blocks used to be formatted using the "default" behavior, and would
thus be mistaken for function calls followed by blocks: this could lead
to unexpected inlining of the block and extra line-break before the
opening brace.

They are now formatted similarly to `@autoreleasepool` blocks, as
expected:

  @synchronized(self) {
      f();
  }

Reviewers: krasimir, djasper, klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D43114

llvm-svn: 326191