[MC] - Don't assert when non-english characters are used.

I found that llvm-mc does not like non-english characters even in comments,
which it tries to tokenize.

Problem happens because of functions like isdigit(), isalnum() which takes
int argument and expects it is not negative.
But at the same time MCParser uses char* to store input buffer poiner, char has signed value,
so it is possible to pass negative value to one of functions from above and
that triggers an assert. 
Testcase for demonstration is provided.

To fix the issue helper functions were introduced in StringExtras.h

Differential revision: https://reviews.llvm.org/D38461

llvm-svn: 314883