[Clang][Emscripten]: Don't default to -fvisibility=hidden

[sbc100]

Summary

Emscripten wants -fvisibility=default by default. See:
https://github.com/emscripten-core/emscripten/pull/15413

For more disussion around what the default should be see
https://github.com/WebAssembly/tool-conventions/issues/176.

clang/lib/Driver/ToolChains/Clang.cpp

if (Value.getAsInteger(10, Mregparm)) {
             << A->getOption().getName() << Value;
       }
     }
   }
 }
 
 void Clang::AddWebAssemblyTargetArgs(const ArgList &Args,
                                      ArgStringList &CmdArgs) const {
-  // Default to "hidden" visibility.
-  if (!Args.hasArg(options::OPT_fvisibility_EQ,
+  const llvm::Triple &T = getToolChain().getTriple();
+  // Default to "hidden" visibility for WebAssembly targets except
+  // for emscripten which uses the default for compatibility with
+  // existing code.
+  if (T.getOS() != llvm::Triple::Emscripten &&
+      !Args.hasArg(options::OPT_fvisibility_EQ,
                    options::OPT_fvisibility_ms_compat)) {
     CmdArgs.push_back("-fvisibility");
     CmdArgs.push_back("hidden");
   }
 }
 
 void Clang::AddVETargetArgs(const ArgList &Args, ArgStringList &CmdArgs) const {
   // Floating point operations and argument passing are hard.

clang/test/Driver/wasm-toolchain.c

 // A basic clang -cc1 command-line. WebAssembly is somewhat special in
 // enabling -fvisibility=hidden by default.
 
 // RUN: %clang %s -### -no-canonical-prefixes -target wasm32-unknown-unknown 2>&1 \
 // RUN:   | FileCheck -check-prefix=CC1 %s
 // CC1: clang{{.*}} "-cc1" "-triple" "wasm32-unknown-unknown" {{.*}} "-fvisibility" "hidden" {{.*}}
 
 // Ditto, but ensure that a user -fvisibility=default disables the default
 // -fvisibility=hidden.
 
 // RUN: %clang %s -### -target wasm32-unknown-unknown -fvisibility=default 2>&1 \
 // RUN:   | FileCheck -check-prefix=FVISIBILITY_DEFAULT %s
 // FVISIBILITY_DEFAULT-NOT: hidden
 
+// Emscripten is different in that it defaults to default, like other non-WebAssembly targets.
+// RUN: %clang %s -### -target wasm32-unknown-emscripten 2>&1 \
+// RUN:   | FileCheck -check-prefix=FVISIBILITY_DEFAULT %s
+
 // A basic C link command-line with unknown OS.
 
 // RUN: %clang -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=LINK %s
 // LINK: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK: wasm-ld{{.*}}" "-L/foo/lib" "crt1.o" "[[temp]]" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
 
 // A basic C link command-line with optimization with unknown OS.