[clang-tidy] Add new hicpp-multiway-paths-covered check for missing branches

Summary:
This check searches for missing `else` branches in `if-else if`-chains and
missing `default` labels in `switch` statements, that use integers as condition.

It is very similar to -Wswitch, but concentrates on integers only, since enums are
already covered.

The option to warn for missing `else` branches is deactivated by default, since it is
very noise on larger code bases.

Running it on LLVM:
{F5354858} for default configuration
{F5354866} just for llvm/lib/Analysis/ScalarEvolution.cpp, the else-path checker is very noisy!

Reviewers: alexfh, aaron.ballman, hokein

Reviewed By: aaron.ballman

Subscribers: lebedev.ri, Eugene.Zelenko, cfe-commits, mgorny, JDevlieghere, xazax.hun

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D37808

llvm-svn: 318600