[clang] [OpenACC][CIR] Implement beginning of 'copy' lowering for compute con… (PR #140304)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Alright, I've done all as requested.  Would love a 2nd review/approval from Bruno.

@bcardosolopes : If it is alright, I'd like to just do the OpenACC clause refactor in a followup patch.  I'll work on it 'next', immediately after this one.  I'd originally done the visitor, then realized it needed to be accessed from 3 places.  

I should have just defined a single 'emit' function and moved it all to a .h file, but it wasn't clear how 'templaty' I had to be.  In the end, the 'emit' visitor needs to be constructed with a finite number of types, so I can use explicit specializations to handle it.

https://github.com/llvm/llvm-project/pull/140304