[WebAssembly] Make __attribute__((used)) not imply export.

Add an WASM_SYMBOL_NO_STRIP flag, so that __attribute__((used)) doesn't
need to imply exporting. When targeting Emscripten, have
WASM_SYMBOL_NO_STRIP imply exporting.

Differential Revision: https://reviews.llvm.org/D62542

llvm-svn: 370415
diff --git a/llvm/lib/MC/MCWasmStreamer.cpp b/llvm/lib/MC/MCWasmStreamer.cpp
index 86fa721..e7e96ec 100644
--- a/llvm/lib/MC/MCWasmStreamer.cpp
+++ b/llvm/lib/MC/MCWasmStreamer.cpp
@@ -122,7 +122,7 @@
     break;
 
   case MCSA_NoDeadStrip:
-    Symbol->setExported();
+    Symbol->setNoStrip();
     break;
 
   default: