[clang-tidy] Add a checker for long functions.

As this is very dependent on the code base it has some ways of configuration.
It's possible to pick between 3 modes of operation:

- Line counting: number of lines including whitespace and comments
- Statement counting: number of statements within compoundStmts.
- Branch counter

In addition a threshold can be picked, warnings are only emitted when it is met.
The thresholds can be configured via a .clang-tidy file.

Differential Revision: http://reviews.llvm.org/D4986