Move format internal code from llvm::detail to llvm::support::detail. (#87288)

Some support code, e.g. llvm/Support/Endian.h, uses
llvm::support::detail, but the format-related code uses llvm::detail. On
VS2019, when a C++ file includes both headers, a `detail::` from
`namespace llvm { ... }` becomes ambiguous.

44253a9c breaks TensorFlow and
[JAX](https://github.com/google/jax/actions/runs/8507773013/job/23300219405)
build because of this.

Since llvm::X::detail seems like a cleaner solution and is used in other
places as well (e.g. llvm::yaml::detail), we should probably migrate all
llvm::detail usages to llvm::X::detail.