[clang-tidy] Add a module for the Linux kernel.

Summary:
Now that clang is going to be able to build the Linux kernel again on
x86, and we have gen_compile_commands.py upstream for generating
compile_commands.json, clang-tidy can be used on the Linux kernel
source.

To that end, this commit adds a new clang-tidy module to be used for
checks specific to Linux kernel source. The Linux kernel follows its own
style of C, and it will be useful to separate those checks into their
own module.

This also adds an initial check that makes sure that return values from
the kernel error functions like PTR_ERR and ERR_PTR are checked. It also
makes sure that any functions that directly return values from these
functions are checked.

Subscribers: xazax.hun, gribozavr, Eugene.Zelenko, lebedev.ri, mgorny, jdoerfert, cfe-commits

Tags: #clang, #clang-tools-extra

Reviewers: aaron.ballman, alexfh, hokein, JonasToth

Differential Revision: https://reviews.llvm.org/D59963

llvm-svn: 367071