[WebAssembly] Don't mark lazy symbols as `IsUsedInRegularObj`

This matches the ELF does.  Update the comment in ELF/Symbols.h and
duplicate it in wasm/Symbols.h

This a followup on rL355580 and rL355577.

Differential Revision: https://reviews.llvm.org/D59075

llvm-svn: 355737
diff --git a/lld/wasm/Writer.cpp b/lld/wasm/Writer.cpp
index f5ff302..0082ae3 100644
--- a/lld/wasm/Writer.cpp
+++ b/lld/wasm/Writer.cpp
@@ -950,7 +950,7 @@
   };
 
   for (Symbol *Sym : Symtab->getSymbols())
-    if (!Sym->isLazy() && Sym->IsUsedInRegularObj)
+    if (Sym->IsUsedInRegularObj)
       AddSymbol(Sym);
 
   for (ObjFile *File : Symtab->ObjectFiles) {