[clang][WebAssembly] Initial support for reference type externref in clang

This patch introduces a new type __externref_t that denotes a WebAssembly opaque
reference type. It also implements builtin __builtin_wasm_ref_null_extern(),
that returns a null value of __externref_t. This lays the ground work
for further builtins and reference types.

Differential Revision: https://reviews.llvm.org/D122215