[DebugInfo][Mem2Reg] Assign uninitialized values with annotated locs (#157716)

In PromoteMem2Reg, we perform a DFS over the CFG and track, for each
alloca, its incoming value and its associated incoming DebugLoc, both of
which are taken from stores to that alloca; these values and DebugLocs
are propagated to PHI nodes when new blocks are reached. In the event
that for one incoming edge no store instruction has been seen, we
propagate an UndefValue and an empty DebugLoc to the PHI.

This is a perfectly valid occurrence, and assigning an empty DebugLoc to
the PHI is the correct course of action; therefore, we should pass an
annotated DebugLoc instead, so that in DebugLoc coverage tracking we
correctly do not expect a valid DebugLoc to be present; we generally
mark allocas as having CompilerGenerated locations, so I've chosen to
use the same annotation to represent the uninitialized value of that
alloca.

This change is NFC outside of DebugLoc coverage tracking builds.