[clang-tidy] Add new checker: llvm-prefer-isa-or-dyn-cast-in-conditionals

Summary:
Looks at conditionals and finds cases of ``cast<>``, which will
assert rather than return a null pointer, and ``dyn_cast<>`` where
the return value is not captured. Additionally, finds cases that
match the pattern ``var.foo() && isa<X>(var.foo())``, where the
method is called twice and could be expensive.

.. code-block:: c++

  // Finds cases like these:
  if (auto x = cast<X>(y)) <...>
  if (cast<X>(y)) <...>

  // But not cases like these:
  if (auto f = cast<Z>(y)->foo()) <...>
  if (cast<Z>(y)->foo()) <...>

Reviewers: alexfh, rjmccall, hokein, aaron.ballman, JonasToth

Reviewed By: aaron.ballman

Subscribers: xbolva00, Eugene.Zelenko, mgorny, xazax.hun, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D59802

llvm-svn: 359142