Fix sed -e s@FOO@%/S@ and similar when there's @'s in the working directory

Jenkins sometimes starts a new working directory by appending @2 (or
incrementing the number if the @n suffix is already there). This causes
several clang tests to fail as:
  s@INPUT_DIR@%/S/Inputs@g
gets expanded to the invalid:
  s@INPUT_DIR@/path/to/workdir@2/Inputs@g
                               ~~~~~~~~~~
where the part marked with ~'s is interpreted as the flags. These are
invalid and the test fails.

Previous fixes simply exchanged the @ character for another like | but
that's just moving the problem. Address it by adding an expansion that
escapes the @ character we're using as a delimiter as well as other magic
characters in the replacement of sed's s@@@.

There's still room for expansions to cause trouble though. One I ran into
while testing this was that having a directory called foo@bar causes lots
of `CHECK-NOT: foo` directives to match. There's also things like
directories containing `\1`

utils/lit/lit/TestRunner.py

@@ -1105,6 +1105,20 @@ def getDefaultSubstitutions(test, tmpDir, tmpBase, normalize_slashes=False):
             ('%/T', tmpDir.replace('\\', '/')),
             ])
 
+    # "%{/[STpst]:regex_replacement}" should be normalized like "%/[STpst]" but we're
+    # also in a regex replacement context of a s@@@ regex.
+    def regex_escape(s):
+        s = s.replace('@', '\@')
+        s = s.replace('&', '\&')
+        return s
+    substitutions.extend([
+            ('%{/s:regex_replacement}', regex_escape(sourcepath.replace('\\', '/'))),
+            ('%{/S:regex_replacement}', regex_escape(sourcedir.replace('\\', '/'))),
+            ('%{/p:regex_replacement}', regex_escape(sourcedir.replace('\\', '/'))),
+            ('%{/t:regex_replacement}', regex_escape(tmpBase.replace('\\', '/')) + '.tmp'),
+            ('%{/T:regex_replacement}', regex_escape(tmpDir.replace('\\', '/'))),
+            ])
+
     # "%:[STpst]" are normalized paths without colons and without a leading
     # slash.
     substitutions.extend([