[WebAssembly] Add -lc++-abi when linking C++ programs

[dschuff]

Cool, let's go with D41937, it has a proper diagnostic, and a test!

lib/Driver/ToolChains/WebAssembly.h

private:
   bool UseObjCMixedDispatch() const override;
   bool isPICDefault() const override;
   bool isPIEDefault() const override;
   bool isPICDefaultForced() const override;
   bool IsIntegratedAssemblerDefault() const override;
   bool hasBlocksRuntime() const override;
   bool SupportsProfiling() const override;
   bool HasNativeLLVMSupport() const override;
-  void addClangTargetOptions(const llvm::opt::ArgList &DriverArgs,
-                             llvm::opt::ArgStringList &CC1Args,
-                             Action::OffloadKind DeviceOffloadKind) const override;
+  void
+  addClangTargetOptions(const llvm::opt::ArgList &DriverArgs,
   RuntimeLibType GetDefaultRuntimeLibType() const override;
   CXXStdlibType GetCXXStdlibType(const llvm::opt::ArgList &Args) const override;
+  void AddCXXStdlibLibArgs(const llvm::opt::ArgList &Args,
+                           llvm::opt::ArgStringList &CmdArgs) const override;
   void AddClangSystemIncludeArgs(
       const llvm::opt::ArgList &DriverArgs,
       llvm::opt::ArgStringList &CC1Args) const override;
   void AddClangCXXStdlibIncludeArgs(
       const llvm::opt::ArgList &DriverArgs,
       llvm::opt::ArgStringList &CC1Args) const override;
   std::string getThreadModel() const override;
 

lib/Driver/ToolChains/WebAssembly.cpp

 ToolChain::RuntimeLibType WebAssembly::GetDefaultRuntimeLibType() const {
   return ToolChain::RLT_CompilerRT;
 }
 
 ToolChain::CXXStdlibType WebAssembly::GetCXXStdlibType(const ArgList &Args) const {
   return ToolChain::CST_Libcxx;
 }
 
+void WebAssembly::AddCXXStdlibLibArgs(const ArgList &Args,
+                                      ArgStringList &CmdArgs) const {
+  assert((GetCXXStdlibType(Args) == ToolChain::CST_Libcxx) &&
+         "Only -lc++ (aka libxx) is supported in this toolchain.");
+
+  CmdArgs.push_back("-lc++");
+  CmdArgs.push_back("-lc++abi");
+}
+
 void WebAssembly::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
                                             ArgStringList &CC1Args) const {
   if (!DriverArgs.hasArg(options::OPT_nostdinc))
     addSystemInclude(DriverArgs, CC1Args, getDriver().SysRoot + "/include");
 }
 
 void WebAssembly::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
                                                ArgStringList &CC1Args) const {