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

[Nikita Popov via cfe-commits]

nikic wrote:

For example, the SPIR builtins file has barely more than 50 lines of code but clocks in at 30M instructions (the *whole* of CGBuiltins.cpp was 120M previously).

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