I also fixed __builtin_wasm_ref_null_extern() to generate a diagnostic
when it gets an argument. It seems like `SemaRef.checkArgCount()` has a
bug that makes it unable to check for 0 args.

Remove superfluous header files after refactor in #132252

clang/lib/CodeGen/CGBuiltin.cpp is over 1MB long (>23k LoC), and can
take minutes to recompile (depending on compiler and host system) when
modified, and 5 seconds for clangd to update for every edit. Splitting
this file was discussed in this thread:

   https://discourse.llvm.org/t/splitting-clang-s-cgbuiltin-cpp-over-23k-lines-long-takes-1min-to-compile/

and the idea has received a number of +1 votes, hence this change.