[WebAssemblyLowerEmscriptenEHSjLj] Avoid setting import_name where possible (#128564)
This change effectively reverts 296ccef
(https://reviews.llvm.org/D77192)
Most of these symbols are just normal C symbols that get imported from
wither libcompiler-rt or from emscripten's JS library code. In most
cases it should not be necessary to give them explicit import names.
The advantage of doing this is that we can wasm-ld can/will fail with a
useful error message when these symbols are missing. As opposed to today
where it will simply import them and defer errors until later (when they
are less specific).