[clang][NFC] Clean up `Sema` headers

When various `Sema*.h` and `Sema*.cpp` files were created, cleanup of
`Sema.h` includes and forward declarations was left for the later.
Now's the time. This com

[clang][NFC] Clean up `Sema` headers

When various `Sema*.h` and `Sema*.cpp` files were created, cleanup of
`Sema.h` includes and forward declarations was left for the later.
Now's the time. This commit touches `Sema.h` and Sema components:
1. Unused includes are removed.
2. Unused forward declarations are removed.
3. Missing includes are added (those files are largely IWYU-clean now).
4. Includes were converted into forward declarations where possible.

As this commit focuses on headers, all changes to `.cpp` files were
minimal, and were aiming at keeping everything buildable.

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[PPC] Implement BCD assist builtins (#101390)

Implement BCD assist builtins for XL and GCC compatibility.

GCC compat:
```
unsigned int __builtin_cdtbcd (unsigned int);
unsigned int __builtin_c

[PPC] Implement BCD assist builtins (#101390)

Implement BCD assist builtins for XL and GCC compatibility.

GCC compat:
```
unsigned int __builtin_cdtbcd (unsigned int);
unsigned int __builtin_cbcdtd (unsigned int);
unsigned int __builtin_addg6s (unsigned int, unsigned int);
```

64BIT XL compat:
```
long long __cdtbcd (long long);
long long __cbcdtd (long long);
long long __addg6s (long long source1, long long source2)
```

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[clang][Sema][NFC] remove unreachable return statement (#100642)

The switch now returns in every case so the end return is unreachable.

[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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