[llvm] [X86][NFC] Use compact enum in TargetParser (PR #206088)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> What's the gain from the change? Compile time or code size reduction? It's better if you have benchmark data.

https://github.com/llvm/llvm-project/pull/202044

This is a dynamic-relocation-free string table with uint16_t offsets and uint8_t lengths. `StringLiteral` in static storage duration requires dynamic relocations and is also large (16 bytes).

https://github.com/llvm/llvm-project/pull/206088