[CodeGen] Don't create fake FunctionDecls when generating block/byref

copy/dispose helper functions

We found out that these fake functions would cause clang to crash if the
changes proposed in https://reviews.llvm.org/D98799 were made.

The original patch was reverted in f681fd92
because debug locations were missing in the body of the block byref
helper functions. This patch fixes the bug by calling CreateArtificial
after the calls to StartFunction.

Differential Revision: https://reviews.llvm.org/D104082