[clang] Introduce target-specific `Sema` components (#93179)

This patch introduces `SemaAMDGPU`, `SemaARM`, `SemaBPF`, `SemaHexagon`,
`SemaLoongArch`, `SemaMIPS`, `SemaNVPTX`, `SemaPPC`, `SemaSyste

[clang] Introduce target-specific `Sema` components (#93179)

This patch introduces `SemaAMDGPU`, `SemaARM`, `SemaBPF`, `SemaHexagon`,
`SemaLoongArch`, `SemaMIPS`, `SemaNVPTX`, `SemaPPC`, `SemaSystemZ`,
`SemaWasm`. This continues previous efforts to split Sema up. Additional
context can be found in #84184 and #92682.

I decided to bundle target-specific components together because of their
low impact on `Sema`. That said, their impact on `SemaChecking.cpp` is
far from low, and I consider it a success.

Somewhat accidentally, I also moved Wasm- and AMDGPU-specific function
from `SemaDeclAttr.cpp`, because they were exposed in `Sema`. That went
well, and I consider it a success, too. I'd like to move the rest of
static target-specific functions out of `SemaDeclAttr.cpp` like we're
doing with built-ins in `SemaChecking.cpp` .

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