[Asm] Refactor debug printing of AsmToken

* Move printing from llvm-mc to the AsmToken class, so that it can be used elsewhere.
* Add 5 cases which were missed: BigNum, Comment, HashDirective, Space and
  BackSlash, and remove the default case so that -Wswitch will catch this error
  in future.

This is almost NFC, except for the fact that llvm-mc can now print those 5
tokens in -as-lex mode.

Differential revision: https://reviews.llvm.org/D43936



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326794 91177308-0d34-0410-b5e6-96231b3b80d8