[clang-tidy] Don't ignore default set of checks when a config file is found.

Summary:
This makes clang-tidy merge the default set of checks with the one
provided in the configuration file instead of just using the checks from the
config file. This adds a way to modify the default set of checks while the
previous behavior required to always define the set of checks completely.

Reviewers: djasper

Reviewed By: djasper

Subscribers: curdeius, cfe-commits

Differential Revision: http://reviews.llvm.org/D7434

llvm-svn: 228298