[DebugInfo] Don't reorder DBG_VALUEs when sunk

Fix part of PR43855, resolving a problem that comes from the reapplication
in 001574938e5. If we have two DBG_VALUE insts in a block that specify
the location of the same variable, for example:

   %0 = someinst
   DBG_VALUE %0, !123, !DIExpression()
   %1 = anotherinst
   DBG_VALUE %1, !123, !DIExpression()

if %0 were to sink, the corresponding DBG_VALUE would sink too, past the
next DBG_VALUE, effectively re-ordering assignments. To fix this, I've
added a SeenDbgVars set recording what variable locations have been seen in
a block already (working bottom up), and now flag DBG_VALUEs that would
pass a later DBG_VALUE for the same variable.

NB, this only works for repeated DBG_VALUEs in the same basic block, the
general case involving control flow is much harder, which I've written
up in PR44117.

Differential revision: https://reviews.llvm.org/D70672