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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

The RISCV builtins are generated code; riscv_vector_builtin_cg.inc is 87000 lines.  There's an open bug about this: #88368.

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