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

[mgorny]

Summary

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

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)

[MaskRay]

D122569 (@asavonic) introduced the issue.

[mgorny]

Thanks!

llvm/utils/lit/lit/TestRunner.py

 def expand_glob_expressions(args, cwd):
     result = [args[0]]
     for arg in args[1:]:
         result.extend(expand_glob(arg, cwd))
     return result
 
 
 def quote_windows_command(seq):
-    """
+    r"""
     Reimplement Python's private subprocess.list2cmdline for MSys compatibility
 
     Based on CPython implementation here:
       https://hg.python.org/cpython/file/849826a900d2/Lib/subprocess.py#l422
 
     Some core util distributions (MSys) don't tokenize command line arguments
     the same way that MSVC CRT does. Lit rolls its own quoting logic similar to
     the stock CPython logic to paper over these quoting and tokenization rule
def substituteIfElse(ln):
                 raise ValueError("'%{' is missing for %if substitution")
             cond = ln[: match.start()]
 
             # eat '%{' as well
             ln = ln[match.end() :]
             return cond, ln
 
         def tryParseElse(ln):
-            match = _caching_re_compile("^\s*%else\s*(%{)?").search(ln)
+            match = _caching_re_compile(r"^\s*%else\s*(%{)?").search(ln)
             if not match:
                 return False, ln
             if not match.group(1):
                 raise ValueError("'%{' is missing for %else substitution")
             return True, ln[match.end() :]
 
         def tryParseEnd(ln):
             if ln.startswith("%}"):