[clang-tidy] add option to specify build path

[Sarcasm]

Summary

compile_commands.json is usually generated in the build directory.
Projects like LLVM/Clang enforce out-of-source builds.
This option allow allow such projects to work out of the box, without
moving the compilation database manually.

The naming of the option is similar to the one use by other tools:

clang-{check,modernize,query,rename,tidy} -p=<build_path> <...>

[alexfh]

Looks good!

clang-tools-extra/trunk/clang-tidy/tool/run-clang-tidy.py

parser.add_argument('-header-filter', default=None,
                       'displayed.')
   parser.add_argument('-j', type=int, default=0,
                       help='number of tidy instances to be run in parallel.')
   parser.add_argument('files', nargs='*', default=['.*'],
                       help='files to be processed (regex on path)')
   parser.add_argument('-fix', action='store_true', help='apply fix-its')
   parser.add_argument('-format', action='store_true', help='Reformat code '
                       'after applying fixes')
+  parser.add_argument('-p', dest='build_path',
+                      help='Path used to read a compile command database.')
   args = parser.parse_args()
 
+  db_path = 'compile_commands.json'
+
+  if args.build_path is not None:
+    build_path = args.build_path
+  else:
+    # Find our database
+    build_path = find_compilation_database(db_path)
+
   try:
     invocation = [args.clang_tidy_binary, '-list-checks']
+    invocation.append('-p=' + build_path)
     if args.checks:
       invocation.append('-checks=' + args.checks)
     invocation.append('-')
     print subprocess.check_output(invocation)
   except:
     print >>sys.stderr, "Unable to run clang-tidy."
     sys.exit(1)
 
-  # Find our database.
-  db_path = 'compile_commands.json'
-  build_path = find_compilation_database(db_path)
-
   # Load the database and extract all files.
   database = json.load(open(os.path.join(build_path, db_path)))
   files = [entry['file'] for entry in database]
 
   max_task = args.j
   if max_task == 0:
     max_task = multiprocessing.cpu_count()