[clang-tidy] New argument --language to add_new_check.py

Summary:
Currently, add_new_check.py assumes all checks are for C++ code.

This adds a new argument --language=[LANG] to add_new_check.py
so authors of new checks can specify that the test file should
be in a different language.

For example, authors can pass --language=objc for Objective-C
clang-tidy checks.

Reviewers: hokein, alexfh

Reviewed By: alexfh

Subscribers: Wizard, xazax.hun

Differential Revision: https://reviews.llvm.org/D39141

llvm-svn: 323919