[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

Author: mgorny

llvm/utils/lit/lit/TestRunner.py

@@ -202,7 +202,7 @@ def expand_glob_expressions(args, cwd):
 
 
 def quote_windows_command(seq):
-    """
+    r"""
     Reimplement Python's private subprocess.list2cmdline for MSys compatibility
 
     Based on CPython implementation here:
@@ -1563,7 +1563,7 @@ def tryParseIfCond(ln):
             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):