[clang] Refactor mustprogress handling, add it to all loops in c++11+.

Currently Clang does not add mustprogress to inifinite loops with a
known constant condition, matching C11 behavior. The forward progress
guarantee in C++11 and later should allow us to add mustprogress to any
loop (http://eel.is/c++draft/intro.progress#1).

This allows us to simplify the code dealing with adding mustprogress a
bit.

Reviewed By: aaron.ballman, lebedev.ri

Differential Revision: https://reviews.llvm.org/D96418