[clang-tidy] Add a generic header guard checker + LLVM implementation.

The implementation is split into a generic part and a LLVM-specific part.
Other codebases can implement it with their own style. The specific features
supported are:

- Verification (and fixing) of header guards against a style based on the file path
- Automatic insertion of header guards for headers that are missing them
- A warning when the header guard doesn't enable our fancy header guard optimization
(e.g. when there's an include preceeding the guard)
- Automatic insertion of a comment with the guard name after #endif.

For the LLVM style we disable #endif comments for now, they're not very common
in the codebase. We also only flag headers in the include directories, there
doesn't seem to be a common style outside.

Differential Revision: http://reviews.llvm.org/D4867

llvm-svn: 215548