[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
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