Do not emit prologue_end for line 0 locs if there is a non-zero loc present

This change fixes a bug where the compiler generates a prologue_end
for line 0 locs. That is because line 0 is not associated with any
source location, so there should not be a prolgoue_end at a location
that doesn't correspond to a source location.

There were some LLVM tests that were explicitly checking for line 0
prologue_end's as well since I believe that to be incorrect, I had to
change those tests as well.

Patch by Shubham Rastogi!

Differential Revision: https://reviews.llvm.org/D110740