SMECallAttrs is a new helper class that holds all the SMEAttrs for a
call. The interfaces to query actions needed for the call (e.g. change
streaming mode) have been moved to the SMECallAttrs class.
The main motivation for this change is to make the split between the
caller, callee, and callsite attributes more apparent.
Before this change, we would always merge callsite and callee
attributes. The main reason to do this was to handle indirect calls,
however, we also occasionally used callsite attributes on direct calls
in tests (mainly to avoid creating multiple function declarations). With
this patch, we now explicitly handle indirect calls and disallow
incompatible attributes on direct calls (so this patch is not entirely
an NFC).
Same as #137239, but with a change to avoid inferring SME attributes for
function definitions. This allows stubbing the SME ABI routines in C/C++
(and matches the old behaviour).
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