New -assume-filename=param to check_clang_tidy.py (like clang-format)

Summary:
Currently, check_clang_tidy.py includes logic to select default
clang flags based on the extension of the source filename passed
as the first argument.

Since the source filename might be a temporary or test file with an
arbitrary extension unrelated to the file type, this adds the ability
to override the logic the same way `clang-format`'s -assume-filename=
parameter does.

I included a test with a nonstandard file extension. I confirmed
when I modified the warning message that the new test failed,
and that it passed again when I restored the warning message.

Ran tests with:

% cmake -G Ninja /path/to/llvm
% ninja check-clang-tools

Patch by Ben Hamilton!

Reviewers: hokein, alexfh

Reviewed By: hokein

Subscribers: alexfh

Differential Revision: https://reviews.llvm.org/D38963

llvm-svn: 316066