[clang] [NFC][clang] Split clang/lib/CodeGen/CGBuiltin.cpp into target-specific files (PR #132252)

[Jonathan Thackray via cfe-commits]

jthackray wrote:

@jurahul 

> I see that some of the HLSL builtin code (like `EmitHLSLBuiltinExpr`) ended up in AMDGPU.cpp. Does that seem misplaced?

@jurahul Yes, apologies. Will write another PR to move it.

https://github.com/llvm/llvm-project/pull/132252