[clang-tidy] Enhance modernize-use-auto to casts

Summary:
Extend modernize-use-auto to cases when a variable is assigned with a cast.

e.g.
Type *Ptr1 = dynamic_cast<Type*>(Ptr2);

http://llvm.org/PR25499

Reviewers: angelgarcia, aaron.ballman, klimek, Prazek, alexfh

Subscribers: Prazek, Eugene.Zelenko, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D25316

llvm-svn: 285579