[FileCheck] Fix isalpha/isalnum calls

D79276 caused the following builder to fail:

  http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-win/builds/23489

Specifically, FileCheck dumped stack in the following tests:

  LLVM :: MC/Mips/micromips-jump-pc-region.s
  LLVM :: MC/Mips/mips-jump-pc-region.s

Those tests contained characters encoded as 160 but that render (at
least for me in vim) like a single space (32).  Those characters
appeared between the `#` and `RUN:` on several lines, and D79276
caused FileCheck to process those lines differently: `RUN:` is a
comment directive.  As a result, D79276 caused FileCheck to start
calling is `isalnum` on those characters.

The problem is that FileCheck calls `isalnum` on type `char` without
casting to `unsigned char` first, so it sign-extends 160 beyond what
`unsigned char` or `EOF` can represent.  C says that has undefined
behavior.  This problem is general to FileCheck's prefix parsing and
so exists independently of D79276.

524457ed fixed the above tests.  This patch changes FileCheck to
use LLVM's replacements for `ctype.h` functions, and it adds tests for
cases that are representative with or without D79276.

Reviewed By: jhenderson, thopre, efriedma

Differential Revision: https://reviews.llvm.org/D79810