[WebAssembly] Make EH work with dynamic linking

This makes Wasm EH work with dynamic linking. So far we were only able
to handle destructors, which do not use any tags or LSDA info.

1. This uses `TargetExternalSymbol` for `GCC_except_tableN` symbols,
   which points to the address of per-function LSDA info. It is more
   convenient to use than `MCSymbol` because it can take additional
   target flags.

2. When lowering `wasm_lsda` intrinsic, if PIC is enabled, make the
   symbol relative to `__memory_base` and generate the `add` node. If
   PIC is disabled, continue to use the absolute address.

3. Make tag symbols (`__cpp_exception` and `__c_longjmp`) undefined in
   the backend, because it is hard to make it work with dynamic
   linking's loading order. Instead, we make all tag symbols undefined
   in the LLVM backend and import it from JS.

4. Add support for undefined tags to the linker.

Companion patches:
- https://github.com/WebAssembly/binaryen/pull/4223
- https://github.com/emscripten-core/emscripten/pull/15266

Reviewed By: sbc100

Differential Revision: https://reviews.llvm.org/D111388

lld/wasm/InputFiles.cpp

@@ -663,6 +663,14 @@ Symbol *ObjFile::createUndefined(const WasmSymbol &sym, bool isCalledDirectly) {
     return symtab->addUndefinedTable(name, sym.Info.ImportName,
                                      sym.Info.ImportModule, flags, this,
                                      sym.TableType);
+  case WASM_SYMBOL_TYPE_TAG:
+    if (sym.isBindingLocal())
+      return make<UndefinedTag>(name, sym.Info.ImportName,
+                                sym.Info.ImportModule, flags, this,
+                                sym.Signature);
+    return symtab->addUndefinedTag(name, sym.Info.ImportName,
+                                   sym.Info.ImportModule, flags, this,
+                                   sym.Signature);
   case WASM_SYMBOL_TYPE_SECTION:
     llvm_unreachable("section symbols cannot be undefined");
   }