[flang][runtime] OPEN write-only files

[klausler]

Summary

If a file being opened with no ACTION= is write-only then cope with
it rather than defaulting prematurely to treating it as read-only.

flang/runtime/file.cpp

if (status != OpenStatus::Old) {
       flags |= O_CREAT;
     }
     if (status == OpenStatus::New) {
       flags |= O_EXCL;
     } else if (status == OpenStatus::Replace) {
       flags |= O_TRUNC;
     }
     if (!action) {
-      // Try to open read/write, back off to read-only on failure
+      // Try to open read/write, back off to read-only or even write-only
+      // on failure
       fd_ = ::open(path_.get(), flags | O_RDWR, 0600);
       if (fd_ >= 0) {
         action = Action::ReadWrite;
       } else {
+        fd_ = ::open(path_.get(), flags | O_RDONLY, 0600);
+        if (fd_ >= 0) {
           action = Action::Read;
+        } else {
+          action = Action::Write;
+        }
       }
     }
     if (fd_ < 0) {
       switch (*action) {
       case Action::Read:
         flags |= O_RDONLY;
         break;
       case Action::Write: