[clang-tidy] Handle c-style casts from/to enums.

Summary: Convert c-style casts between integral end enum types to static_cast<>.

Reviewers: klimek

Reviewed By: klimek

Subscribers: curdeius, cfe-commits

Differential Revision: http://reviews.llvm.org/D5558

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@218760 91177308-0d34-0410-b5e6-96231b3b80d8