[clang lint] Support .mm and .cpp test files, let clang decide which language to use

Summary: [clang lint] Support .mm and .cpp test files, let clang decide which language to use

Test Plan:
Tested on fbobjc.

export CLANG_PLUGINS_EXTRA_REPO=extra-repo-example
make -C analyze test

Reviewers: irp, yiding

Reviewed By: yiding

Subscribers: mathieubaudet

Differential Revision: https://phabricator.fb.com/D1463469