[ExecutionEngine] Add a jitTargetAddressToFunction utility function.

jitTargetAddressToFunction takes a JITTargetAddress and returns a pointer of
the given function pointer type suitable for calling to invoke the function
at the target address.

jitTargetAddressToFunction currently behaves the same as
jitTargetAddressToPointer, but in the near future will be updated to perform
pointer signing on architectures that require it (e.g. arm64e). For this
reason it should always be preferred when generating callable pointers for
JIT'd functions.