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

[Tarun Prabhu via cfe-commits]

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tarunprabhu wrote:

Consider replacing `#0` with `#{{[0-9]+}}`. While the attributes are almost certainly going to be in `#0` for such a small function, you don't actually _require_ this to match `#0`, so you it might be better to let it match any number. 

Also, if the `tune-cpu` attribute is expected to be on the same line, you could use `CHECK-SAME`. 

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