[clang] [flang] [mlir] [flang] Add basic -mtune support (PR #95043)

[Tarun Prabhu via cfe-commits]

https://github.com/tarunprabhu commented:

Is there a way to test that the attribute is lowered to FIR and perhaps also on to LLVM? `flang/test/Lower/` has some tests which check for the presence of certain constructs in the LLVM IR. Could something be written similar to those?

https://github.com/llvm/llvm-project/pull/95043