[WebAssembly] Support -stdlib=libc++ switch

Referenced implementation from Fuchsia and Darwin Toolchain.
Still only support CST_Libcxx.  Now checks that the argument
is really '-stdlib=libc++', and display error.

Also, now will pass -lc++ and -lc++abi to the linker.

Patch by Patrick Cheng!

Differential Revision: https://reviews.llvm.org/D41937

llvm-svn: 322382
diff --git a/clang/lib/Driver/ToolChains/WebAssembly.h b/clang/lib/Driver/ToolChains/WebAssembly.h
index 8784e12..cdbb34f 100644
--- a/clang/lib/Driver/ToolChains/WebAssembly.h
+++ b/clang/lib/Driver/ToolChains/WebAssembly.h
@@ -62,6 +62,8 @@
   void AddClangCXXStdlibIncludeArgs(
       const llvm::opt::ArgList &DriverArgs,
       llvm::opt::ArgStringList &CC1Args) const override;
+  void AddCXXStdlibLibArgs(const llvm::opt::ArgList &Args,
+                           llvm::opt::ArgStringList &CmdArgs) const override;
   std::string getThreadModel() const override;
 
   const char *getDefaultLinker() const override {