[clang] [WebAssembly,clang] Add __builtin_wasm_test_function_pointer_signature (PR #150201)

[Hood Chatham via cfe-commits]

hoodmane wrote:

Other builtins with dynamic return type:
#### `__builtin_call_with_static_chain`  
> This builtin is only available for C. This builtin can be used to call Go closures from C.

#### `__builtin_choose_expr`
> This built-in function is analogous to the ? : operator in C, except that the expression returned has its type unaltered by promotion rules. Also, the built-in function does not evaluate the expression that is not chosen. For example, if const_exp evaluates to true, exp2 is not evaluated even if it has side-effects.
>
> This built-in function can return an lvalue if the chosen argument is an lvalue."

https://github.com/llvm/llvm-project/pull/150201