[llvm] [llvm] Remove `inline constexpr` from AArch64TargetParser.h (PR #71602)

[Rob Suderman via llvm-commits]

rsuderman wrote:

> Does using `StringLiteral` instead of `StringRef` in the struct help? It has a more specialized constructor for constexpr.

I attempted to replace with `StringLiteral` instead of StringRef however some of the constructors use empty list initializers which is not supported by the `StringLiteral` constructors.

https://github.com/llvm/llvm-project/pull/71602