[WebAssembly][NFC] Add test case for functype emission

This test aims to demonstrate the WebAssembly backend's behaviour around
emission of the .functype directive. It covers defined and declared
functions as well as libcalls.

It currently fails to emit functypes for all defined functions at the
head of the file, causing issues with the type checker
<https://github.com/llvm/llvm-project/issues/54022>. The patch in
<https://reviews.llvm.org/D122134> is a proposal to fix this issue.