[WebAssembly] Enable -fvisibility=hidden by default.

This, along with many things in the WebAssembly target, is experimental.
Feedback is welcome.

llvm-svn: 257006
diff --git a/clang/lib/Driver/Tools.h b/clang/lib/Driver/Tools.h
index 168662f..6c67819 100644
--- a/clang/lib/Driver/Tools.h
+++ b/clang/lib/Driver/Tools.h
@@ -82,6 +82,8 @@
                         llvm::opt::ArgStringList &CmdArgs) const;
   void AddHexagonTargetArgs(const llvm::opt::ArgList &Args,
                             llvm::opt::ArgStringList &CmdArgs) const;
+  void AddWebAssemblyTargetArgs(const llvm::opt::ArgList &Args,
+                                llvm::opt::ArgStringList &CmdArgs) const;
 
   enum RewriteKind { RK_None, RK_Fragile, RK_NonFragile };