[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
This allows us to simplify the code dealing with adding mustprogress a
Reviewed By: aaron.ballman, lebedev.ri
Differential Revision: https://reviews.llvm.org/D96418