clang-format: NFC. Add a function to test whether an annotated line

starts with a given sequence of tokens. Only the one-token version is
used yet, but other usages are coming up momentarily.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@239892 91177308-0d34-0410-b5e6-96231b3b80d8