[clang] [clang][Headers] add endian.h (PR #186032)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> This PR draws on that implementation to add a clang header so applications on platforms without it can make use of this header.

I could use a bit more background. It's a POSIX header, so a POSIX implementation should provide it. Providing it on non-POSIX systems leads to the question of why *Clang* (specifically) should vend this header.

https://github.com/llvm/llvm-project/pull/186032