[CIR] Upstream global variable replacement (#184686)
This change upstreams the CIR implementation of global variable
replacement. When we get a call to get or create a global variable using
a type that does not match the previous type for a variable of the same
name, we need to replace the old definition with the new one. In classic
codegen that was as simple as replaceAllUses+eraseFromParent, but in CIR
because we have typed pointers, we need to visit the uses and update
them with bitcasts to reflect the new type.
