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.
Just noticed: the list of the languages is also used in the documentation of the corresponding option below. Maybe make this a map of language -> extension and feed its keys to the choices argument of parser.add_argument?
Sorry for not noticing this earlier.