[WebAssembly] Pass through --undefined to Wasm LLD

This is a follow-on to D40724 (Wasm entrypoint changes #1,
add `--undefined` argument to LLD).

Patch by Nicholas Wilson

Differential Revision: https://reviews.llvm.org/D40739

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@319623 91177308-0d34-0410-b5e6-96231b3b80d8

Author: sbc100

lib/Driver/ToolChains/WebAssembly.cpp

@@ -47,6 +47,7 @@ void wasm::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     CmdArgs.push_back("--strip-all");
 
   Args.AddAllArgs(CmdArgs, options::OPT_L);
+  Args.AddAllArgs(CmdArgs, options::OPT_u);
   ToolChain.AddFilePathLibArgs(Args, CmdArgs);
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles))