[DebugInfo][Docs] Document how dbg.value intrinsics are interpreted in optimized code

This patch adds a section, ``Object lifetime in optimized code'', that
documents how such intrinsics are supposed to be handled. It sets out some of
the principles of how they specify variable locations, and how long those
locations are valid for.

This patch also documents one of the objectives behind the variable-location
design, that we should never allow the debugger to observe a state of the
program that would not have appeared without optimization.

Differential Revision: https://reviews.llvm.org/D58726

llvm-svn: 356041