[Reassociate] Use a reference to DataLayout instead of copying the underlying string data (NFC) (#128269)

I noticed this when looking at all allocations by clang. For a medium
sized file this was around 6000 calls to operator new, although i
suspect there were more allocations in total as the SmallVectors in
DataLayout may have their own allocations in some cases.

In a follow-up i'm tempted to make the DataLayout copy constructor
private, to avoid this in future. There are a few tests which copy the
DataLayout, and perhaps need to (I didn't check yet), but we could
provide a clone() method for them if needed. Its only accidental copying
I think we should consider avoiding, not people who really do need to
copy it for reasons.

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