[utils] Fix PathSanitizingFileCheck to support paths with "weird" chars

When absolute paths that need to be substituted by PathSanitizingFileCheck contained characters that could be part of regexp syntax, substitution didn't actually take place.

For example, substitution like BUILD_DIR='/Users/johnsmith/swift-source/Ninja-RelWithDebInfoAssert+swift-DebugAssert' didn't result in anything because of the '+' in the path.

This is an attempt to fix it.

This commit also adds the --dry-run flag to PathSanitizingFileCheck which was used to detect and debug this issue, but why not let it stay.

Author: broadwaylamb

utils/PathSanitizingFileCheck

@@ -56,6 +56,12 @@ constants.""")
              "available if Windows compatibility is enabled.",
         action="store_true")
 
+    parser.add_argument(
+        "--dry-run",
+        help="Apply the replacements to the input and print the result "
+             "to standard output",
+        action="store_true")
+
     args, unknown_args = parser.parse_known_args()
 
     if args.enable_windows_compatibility:
@@ -70,19 +76,25 @@ constants.""")
 
     for s in args.sanitize_strings:
         replacement, pattern = s.split('=', 1)
-        # We are replacing the Unix path separators in the paths passed as
-        # arguments with a broader pattern to also allow forward slashes and
-        # double escaped slashes in the result that we are checking. Sigh.
-        stdin = re.sub(re.sub(r'/', slashes_re, pattern), replacement, stdin)
-
-    p = subprocess.Popen(
-        [args.file_check_path] + unknown_args, stdin=subprocess.PIPE)
-    stdout, stderr = p.communicate(stdin)
-    if stdout is not None:
-        print(stdout)
-    if stderr is not None:
-        print(stderr, file=sys.stderr)
-    return p.wait()
+        # Since we want to use pattern as a regex in some platforms, we need
+        # to escape it first, and then replace the escaped slash
+        # literal (r'\\/') for our platform-dependent slash regex.
+        stdin = re.sub(re.sub(r'\\/', slashes_re, re.escape(pattern)),
+                       replacement,
+                       stdin)
+
+    if args.dry_run:
+        print(stdin)
+        return 0
+    else:
+        p = subprocess.Popen(
+            [args.file_check_path] + unknown_args, stdin=subprocess.PIPE)
+        stdout, stderr = p.communicate(stdin)
+        if stdout is not None:
+            print(stdout)
+        if stderr is not None:
+            print(stderr, file=sys.stderr)
+        return p.wait()
 
 
 if __name__ == '__main__':