Fixed sys::findProgramByName (Windows) to accept files with point in name

[grimar]

Summary

Problem was in SearchPathW function that does not attach an extension if file already has one.
That does not work for executables like ld.lld2 for example which requre to has .exe extension but SearchPath thinks that its "lld2".
So for such cases I added the extension manually and ignore the extension argument.

Not sure who should review it.

[rnk]

I think we can get away with always manually appending the extension ourselves. We don't need any conditional control flow.

[grimar]

Removed branch (review comment addressed).

[rnk]

lgtm

Thanks!

Program.inc

ErrorOr<std::string> sys::findProgramByName(StringRef Name,
   DWORD Len = MAX_PATH;
   for (StringRef Ext : PathExts) {
     SmallVector<wchar_t, MAX_PATH> U16Ext;
     if (std::error_code EC = windows::UTF8ToUTF16(Ext, U16Ext))
       return EC;
 
     do {
       U16Result.reserve(Len);
-      Len = ::SearchPathW(Path, c_str(U16Name),
-                          U16Ext.empty() ? nullptr : c_str(U16Ext),
+      // Lets attach the extension manually. That is needed for files
+      // with a point in name like aaa.bbb. SearchPathW will not add extension
+      // from its argument to such files because it thinks they already had one.
+      SmallVector<wchar_t, MAX_PATH> U16NameExt;
+      if (std::error_code EC =
+              windows::UTF8ToUTF16(Twine(Name + Ext).str(), U16NameExt))
+        return EC;
+
+      Len = ::SearchPathW(Path, c_str(U16NameExt), nullptr,
                           U16Result.capacity(), U16Result.data(), nullptr);
     } while (Len > U16Result.capacity());
 
     if (Len != 0)
       break; // Found it.
   }
 
   if (Len == 0)