[clang] [Clang][AArch64] Extract get target CPU by triple (NFC) (PR #179097)

[David Sherwood via cfe-commits]

================
@@ -93,6 +69,34 @@ std::string aarch64::getAArch64TargetCPU(const ArgList &Args,
   return "generic";
 }
 
+/// \return the (LLVM) name of the AArch64 cpu we are
----------------
david-arm wrote:

nit: Probably good to reformat these comments before merging, i.e.

```
/// \return the (LLVM) name of the AArch64 cpu we are targeting. Set \p A to the
/// Arg corresponding to the -mcpu argument if it is provided, or to nullptr
/// otherwise.
```

https://github.com/llvm/llvm-project/pull/179097