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

[sbc100]

Summary

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.

[dschuff]

lld/wasm/Driver.cpp

187

this bit looks separate from the 'reproduce' parts, and the commit message only indicates about this. were they intended to be separate CLs?

[sbc100]

• split

lld/wasm/Driver.cpp

187

Sorry, yes, two changes in one.. split them up now.

lld/test/wasm/driver.s

 # RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown -o %t.o %s
 
 .globl  _start
 _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
 
 # RUN: not wasm-ld %t.o 2>&1 | FileCheck -check-prefix=OUT %s
 # OUT: error: no output file specified
 
 # RUN: not wasm-ld 2>&1 | FileCheck -check-prefix=BOTH %s
 # BOTH:     error: no input files

lld/wasm/Driver.cpp

opt::InputArgList WasmOptTable::parse(ArrayRef<const char *> argv) {
   opt::InputArgList args = this->ParseArgs(vec, missingIndex, missingCount);
 
   // Expand response files (arguments in the form of @<filename>)
   // and then parse the argument again.
   cl::ExpandResponseFiles(saver(), getQuotingStyle(args), vec);
   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;
 }
 
 // Currently we allow a ".imports" to live alongside a library. This can
 // be used to specify a list of symbols which can be undefined at link
 // time (imported from the environment.  For example libc.a include an