[MC] Add llvm_unreachable to toString to fix compile time warning.

Without this change, GCC 7 raises the warning below:
        control reaches end of non-void function

Reviewers: sbc100, andreadb

Reviewed By: andreadb

Differential Revision: https://reviews.llvm.org/D46304

llvm-svn: 331255

llvm/lib/MC/WasmObjectWriter.cpp

@@ -47,9 +47,8 @@ static std::string toString(wasm::WasmSymbolType type) {
     return "WASM_SYMBOL_TYPE_DATA";
   case wasm::WASM_SYMBOL_TYPE_SECTION:
     return "WASM_SYMBOL_TYPE_SECTION";
-  default:
-    llvm_unreachable("unexpected kind");
   }
+  llvm_unreachable("unknown symbol type");
 }
 
 static std::string relocTypetoString(uint32_t type) {