[Wasm][Clang] Support for `__externref_t*`?

A year ago I opened a draft pull request adding `__externref_t*` support:
https://github.com/llvm/llvm-project/pull/163610
The idea would be that llvm gains a blessed `__externref_table` of externrefs, the analogue of `__indirect_function_table`. Dereferencing an externref pointer would turn into a `table_get` operation on that table. The table could have a stack region and a heap region. If a pointer to an externref is taken inside of a function, the externref could spill to the stack, so we would bump the stack pointer down 1, use a `table_set` to assign the externref into the table, and then the address of the externref would be the table index. In the epilogue, we'd use a `𝗍𝖺𝖻𝗅𝖾.𝖿𝗂𝗅l` instruction to null out the stack references and then restore the stack pointer. For heap references, we could have `__externref_t* table_alloc(void)` and `void table_free(__externref_t*)`.

This would allow graceful handling of:
* externref in a struct
* externref arrays
* externref in a varargs
* functions with multiple externref return values
with the same ergonomics as normal pointers. Currently all of this stuff works, but the ergonomics are poor and each library has its own ad-hoc handling of these problems so there is no globally agreed-upon notion of an externref pointer.

The biggest problem is that currently `static __externref_t blah[];` declares a table, and a table is represented in the clang IR as an array type. Because arrays freely decay to pointers, this means that clang currently interprets `__externref_t*` as coming from a decayed table and representing also a table. We will need some way to migrate the current behavior to the new behavior. I think tables ideally should be declared with an attribute e.g., `__attribute__((wasmtable)) __externref_t table[];` so that pointers can be reserved for things that admit arithmetic, dereferencing, etc.