[WebAssembly] Error out on invalid personality functions

Without explicitly checking and erroring out, an invalid personality
function, which is not `__gxx_wasm_personality_v0`, caused
a segmentation fault down the line because `WasmEHFuncInfo` was not
created. This explicitly checks the validity of personality functions in
functions with EH pads and errors out explicitly with a helpful error
message. This also adds some more assertions to ensure `WasmEHFuncInfo`
is correctly created and non-null.

Invalid personality functions wouldn't be generated by our Clang, but
can be present in handwritten ll files, and more often, in files
transformed by passes like `metarenamer`, which is often used with
`bugpoint` to simplify names in `bugpoint`-reduced files.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D152203