[PATCH] WIP: AddDiscriminators: Check more carefully for equivalent locations

[Duncan P. N. Exon Smith]

Stop using `DILocation::atSameLineAs()` to check whether two locations
are equivalent.  This check is too weak: it ignores any column info that
might be present, and ignores the scope/inlined-at chains.

Instead, just compare the pointers for equality.  If the pointers are
different here, we'll get different entries in the line table already.

I haven't updated testcases yet (there's one each failing in LLVM and
clang that would need to be rewritten), but basically: is there any
reason to add a discriminator if we'd *already* be getting different
entries in the line table?  (Is there a broken consumer we need to
support that relies on the discriminator field?)

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