[clang-tidy] Fix some false positives in google-readability-casting

Summary:
Ignore C-style casts in extern "C" {} sections. Be more careful when
detecting redundant casts between typedefs to the same type - emit a more
specific warning and don't automatically fix them.

Reviewers: klimek

Reviewed By: klimek

Subscribers: curdeius, cfe-commits

Differential Revision: http://reviews.llvm.org/D7247

llvm-svn: 227444