[clang-tidy] add check cppcoreguidelines-pro-type-static-cast-downcast

Summary:
This check flags all usages of static_cast, where a base class is casted
to a derived class.
In those cases, a fixit is provided to convert the cast to a
dynamic_cast.

Use of these casts can violate type safety and cause the program to
access a variable that is actually of type X to be accessed as if it
were of an unrelated type Z.

This rule is part of the "Type safety" profile of the C++ Core
Guidelines, see
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#-type2-dont-use-static_cast-downcasts-use-dynamic_cast-instead

Depends on D13313

Reviewers: alexfh, sbenza, bkramer, aaron.ballman

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D13368

llvm-svn: 250098