[lit] Use raw strings for backslash escapes to fix SyntaxWarnings

Use raw strings instead of regular strings when escapes are used for
regex matches or arbitrary letters rather than C-style characters.
This fixes `SyntaxWarning`s:

```
TestRunner.py:205: SyntaxWarning: invalid escape sequence '\c'
  """
TestRunner.py:1566: SyntaxWarning: invalid escape sequence '\s'
  match = _caching_re_compile("^\s*%else\s*(%{)?").search(ln)
```

Differential Revision: https://reviews.llvm.org/D158356