[clang-tidy] Add -extra-arg and -extra-arg-before to run-clang-tidy.py

Summary:
These flags allow specifying extra arguments to the tool's command
line which don't appear in the compilation database.

Reviewers: alexfh

Differential Revision: https://reviews.llvm.org/D28334

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@292415 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed