[WebAssembly] Don't default to ELF in the triple.

WebAssembly doesn't yet have a specified binary format, and it may not
end up being ELF, so we don't want the Triple class defaulting to ELF
for it at this time.

llvm-svn: 245254

Author: Dan Gohman

llvm/lib/Support/Triple.cpp

@@ -539,6 +539,11 @@ static Triple::ObjectFormatType getDefaultFormat(const Triple &T) {
     if (T.isOSDarwin())
       return Triple::MachO;
     return Triple::ELF;
+
+  case Triple::wasm32:
+  case Triple::wasm64:
+    // Unknown for now, until an object format is specified.
+    return Triple::UnknownObjectFormat;
   }
 
   if (T.isOSDarwin())