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

[Alexander Richardson via cfe-commits]

arichardson wrote:

This makes a lot of sense to me, just wondering why you chose `clang/lib/CodeGen/BuiltinTargets/` rather than `clang/lib/CodeGen/TargetBuiltins/`? The former shounds like it contains the list of all builtin targets, rather than the codegen for target-specific builtins.

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