Auto merge of #128812 - nnethercote:shrink-TyKind-FnPtr, r=compiler-errors

Shrink `TyKind::FnPtr`.

By splitting the `FnSig` within `TyKind::FnPtr` into `FnSigTys` and `FnHeader`, which can be packed more efficiently. This reduces the size of the hot `TyKind` type from 32 bytes to 24 bytes on 64-bit platforms. This reduces peak memory usage by a few percent on some benchmarks. It also reduces cache misses and page faults similarly, though this doesn't translate to clear cycles or wall-time improvements on CI.

r? `@compiler-errors`

Author: bors