[lld][WebAssemlby] Check for command line flags with missing arguments

I'm really not sure how this was overlooked when we first ported lld
to Wasm.  The upstream code in the ELF backend has these two lines but
for some reason they never make it into the Wasm version.

Differential Revision: https://reviews.llvm.org/D126497

lld/test/wasm/driver.s

@@ -5,6 +5,9 @@ _start:
   .functype _start () -> ()
   end_function
 
+# RUN: not wasm-ld %t -o 2>&1 | FileCheck --check-prefix=NO_O_VAL %s
+# NO_O_VAL: error: -o: missing argument
+
 # RUN: not wasm-ld -o %t.exe 2>&1 | FileCheck -check-prefix=IN %s
 # IN: error: no input files
 

lld/wasm/Driver.cpp

@@ -184,6 +184,9 @@ opt::InputArgList WasmOptTable::parse(ArrayRef<const char *> argv) {
   args = this->ParseArgs(vec, missingIndex, missingCount);
 
   handleColorDiagnostics(args);
+  if (missingCount)
+    error(Twine(args.getArgString(missingIndex)) + ": missing argument");
+
   for (auto *arg : args.filtered(OPT_UNKNOWN))
     error("unknown argument: " + arg->getAsString(args));
   return args;