clang-format: Fix unary operator detection in corner case.

Before:
  decltype(* ::std::declval<const T &>()) void F();

After:
  decltype(*::std::declval<const T &>()) void F();

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216724 91177308-0d34-0410-b5e6-96231b3b80d8