[clang-format] Replace unordered_set with an array

Summary: This replaces an unordered_set from r322690 with an array and binary search.

Reviewers: bkramer, benhamilton

Reviewed By: bkramer, benhamilton

Subscribers: jolesiak, benhamilton, klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D42189

llvm-svn: 322749