[IRBuilder][RS4GC] Require FunctionCallee when creating statepoint

This makes the statepoint methods in IRBuilder accept a
FunctionCallee, which carries both the callee and function type.
This is used to add the elementtype attribute to the statepoint call.

RS4GC requires an additional tweak to actually preserve that attribute
-- previously the attributes on the call were completely overwritten.

Differential Revision: https://reviews.llvm.org/D118886