[llvm] [clang] [LoongArch] Support per-global code model attribute for LoongArch (PR #72079)

[WÁNG Xuěruì via cfe-commits]

xen0n wrote:

I think we should initially only support the attribute on LoongArch. GCC's `model` attribute is only ever supported on ia64 and loongarch, yet the patch currently seems to allow other architectures to use this attribute, with LLVM code model names accepted -- this may or may not be what the targets actually want, if they decide to add support for the attribute in GCC.

https://github.com/llvm/llvm-project/pull/72079