Fix codegen of consteval functions returning an empty class, and related issues (#93115)

Fix codegen of consteval functions returning an empty class, and related
issues

If a class is empty, don't store it to memory: the store might overwrite
useful data. Similarly, if a class has tail padding that might overlap
other fields, don't store the tail padding to memory.

The problem here turned out a bit more general than I initially thought:
basically all uses of EmitAggregateStore were broken. Call lowering had
a method that did mostly the right thing, though: CreateCoercedStore.
Adapt CreateCoercedStore so it always does the conservatively right
thing, and use it for both calls and ConstantExpr.

Also, along the way, fix the "overlap" bit in AggValueSlot: the bit was
set incorrectly for empty classes in some cases.

Fixes #93040.

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