[clang] [SystemZ][Clang]Use the function name from the source for name PPA1 (PR #209215)

[Sean Perry via cfe-commits]

perry-ca wrote:

I'm testing changes as you suggested.  One thing I have noticed is that I need to duplicate much of the code from getMangledNameImpl().  Some of it like the CUDA & multiversion doesn't apply to z/OS other parts like should you mangle or not and (possibly) the unique internal linkage do apply.  Should CGM expose a function like getMangledNameImpl() but under a different name?

https://github.com/llvm/llvm-project/pull/209215