This is a minor patch that addresses concerns about lifetime in D94002.
We need to mention that what's written in LangRef isn't everything about lifetime.start/end
and its semantics depends on the stack coloring algorithm's pattern matching of a stack pointer.
If the stack coloring algorithm cannot conclude that a pointer is a stack-allocated object, the pointer is conservatively
considered as a non-stack one because stack coloring won't take this lifetime into account while assigning addresses.
A reference from alloca to lifetime.start/end is added as well.
Would this alternatively be captured by saying that if ptr is a stack-allocated object, then it *might* have the effect of the original alloca creating a dead object and this intrinsic making it live (but non-deterministically, not doing this and just replacing the content by poison is also allowed)?