[clang-tidy] Fix infinite loop in MisplacedWideningCastCheck.

Summary:
In Release mode, the check was infinite looping over chromium code base.

It seems there is something strange with the creation of the Maps.
I believe the compiler is making some assumption with the implicit conversion from enum <-> int.

By moving the map to a standard switch/cases, we no longer allocate memory and we can keep the same behavior. For a small amount of elements, this is fine.

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18833

llvm-svn: 265679