[clang-format] Make parseUnaryOperator non-recursive, NFCI

Summary:
This patch makes the implementation of parseUnaryOperator non-recursive. We had
a problem with a file starting with tens of thousands of +'es and -'es which
caused clang-format to stack overflow.

Reviewers: bkramer

Reviewed By: bkramer

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D39498

llvm-svn: 317113