clang-format: Don't remove 'unknown' tokens.

In certain macros or incorrect string literals, the token stream can
contain 'unknown' tokens, e.g. a single backslash or a set of empty
ticks. clang-format simply treated them as whitespace and removed them
prior to this patch.

This fixes llvm.org/PR17215

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192490 91177308-0d34-0410-b5e6-96231b3b80d8