[clang-tidy] Add check readability-implicit-bool-cast

Summary:
This is another check that I ported to clang-tidy from colobot-lint tool.

As previously discussed on cfe-dev mailing list, this is one of those
checks that I think is general and useful enough for contribution to
clang-tidy.

This patch contains implementation of check taken from colobot-lint, but
it is extended a great deal, including FixIt hints for automated
refactoring, exhaustive testcases, and user documentation.

Reviewers: sbenza, aaron.ballman, alexfh

Subscribers: Eugene.Zelenko

Differential Revision: http://reviews.llvm.org/D13635

llvm-svn: 251235