[NFC] Pass a reference to CodeGenFunction to methods of LValue and

AggValueSlot

This reapplies 8a5b7c35 after a null
dereference bug in CGOpenMPRuntime::emitUserDefinedMapper.

Original commit message:

This is needed for the pointer authentication work we plan to do in the
near future.

https://github.com/apple/llvm-project/blob/a63a81bd9911f87a0b5dcd5bdd7ccdda7124af87/clang/docs/PointerAuthentication.rst