[WebAssembly] Fix build failures introduced in r305769

This fixes two build failures that only occur in certain
configurations:
- error: unused function 'operator<<'
- error: control reaches end of non-void function

Differential Revision: https://reviews.llvm.org/D34382

llvm-svn: 305770

llvm/lib/Object/WasmObjectFile.cpp

@@ -753,6 +753,7 @@ uint64_t WasmObjectFile::getSymbolValueImpl(DataRefImpl Symb) const {
   case WasmSymbol::SymbolType::DEBUG_FUNCTION_NAME:
     return Sym.ElementIndex;
   }
+  llvm_unreachable("invalid symbol type");
 }
 
 uint32_t WasmObjectFile::getSymbolAlignment(DataRefImpl Symb) const {