[ms] llvm-lib gives a more useful error if no inputs and no output path are provided

[epastor]

Summary

If no inputs and no output path are provided, llvm-lib should produce a useful error.

Before this, it would fail by reading from an unitialized StringRef.

[vvereschaka]

I have tested this patch on the ubuntu builder with enabled expensive checks and no-inputs.test test has passed.

...
PASS: LLVM :: tools/llvm-lib/no-inputs.test (35368 of 37269)
...
Testing Time: 518.63s
  Unsupported Tests:   319
  Expected Passes  : 36789
  Expected Failures:   161

I can't verify the patch on the other builders with the expensive checks - Debian and Windows, but I think they should be fine with these changes.

[vvereschaka]

I see that Harbormaster failed the build on Windows, but there is a lot of error: Connection to server timed out errors. Looks like there is some problem with that build system.

llvm/lib/ToolDrivers/llvm-lib/LibDriver.cpp

 
 class LibOptTable : public opt::OptTable {
 public:
   LibOptTable() : OptTable(InfoTable, true) {}
 };
 
 }
 
-static std::string getOutputPath(opt::InputArgList *Args,
-                                 const NewArchiveMember &FirstMember) {
-  if (auto *Arg = Args->getLastArg(OPT_out))
-    return Arg->getValue();
+static std::string getDefaultOutputPath(const NewArchiveMember &FirstMember) {
   SmallString<128> Val = StringRef(FirstMember.Buf->getBufferIdentifier());
   sys::path::replace_extension(Val, ".lib");
   return std::string(Val.str());
 }
 
 static std::vector<StringRef> getSearchPaths(opt::InputArgList *Args,
                                              StringSaver &Saver) {
   std::vector<StringRef> Ret;
for (auto *Arg : Args.filtered(OPT_INPUT)) {
     // Append a file.
     appendFile(Members, LibMachine, LibMachineSource, MBRef);
 
     // Take the ownership of the file buffer to keep the file open.
     MBs.push_back(std::move(*MOrErr));
   }
 
   // Create an archive file.
-  std::string OutputPath = getOutputPath(&Args, Members[0]);
+  std::string OutputPath;
+  if (auto *Arg = Args.getLastArg(OPT_out)) {
+    OutputPath = Arg->getValue();
+  } else if (!Members.empty()) {
+    OutputPath = getDefaultOutputPath(Members[0]);
+  } else {
+    llvm::errs() << "no output path given, and cannot infer with no inputs\n";
+    return 1;
+  }
   // llvm-lib uses relative paths for both regular and thin archives, unlike
   // standard GNU ar, which only uses relative paths for thin archives and
   // basenames for regular archives.
   for (NewArchiveMember &Member : Members) {
     if (sys::path::is_relative(Member.MemberName)) {
       Expected<std::string> PathOrErr =
           computeArchiveRelativePath(OutputPath, Member.MemberName);
       if (PathOrErr)