[clang-tidy] Add MagnitudeBitsUpperLimit option to bugprone-too-small-loop-variable

Summary:
The bugprone-too-small-loop-variable check often catches loop variables which can represent "big enough" values, so we don't actually need to worry about that this variable will overflow in a loop when the code iterates through a container. For example a 32 bit signed integer type's maximum value is 2 147 483 647 and a container's size won't reach this maximum value in most of the cases.
So the idea of this option to allow the user to specify an upper limit (using magnitude bit of the integer type) to filter out those catches which are not interesting for the user, so he/she can focus on the more risky integer incompatibilities.
Next to the option I replaced the term "positive bits" to "magnitude bits" which seems a better naming both in the code and in the name of the new option.

Reviewers: JonasToth, alexfh, aaron.ballman, hokein

Reviewed By: JonasToth

Subscribers: Eugene.Zelenko, xazax.hun, jdoerfert, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D59870

llvm-svn: 358356