[flang][runtime] INQUIRE(FILE=path, READ/READWRITE/WRITE=x) should be UNKNOWN when unknown

[klausler]

Summary

For nonexistent or inaccessible files, we're returning NO, which is
indeed true, but we should return UNKNOWN instead.

flang/runtime/io-stmt.cpp

bool InquireUnconnectedFileState::Inquire(
   case HashInquiryKeyword("ENCODING"):
   case HashInquiryKeyword("FORMATTED"):
   case HashInquiryKeyword("SEQUENTIAL"):
   case HashInquiryKeyword("STREAM"):
   case HashInquiryKeyword("UNFORMATTED"):
     str = "UNKNONN";
     break;
   case HashInquiryKeyword("READ"):
-    str = MayRead(path_.get()) ? "YES" : "NO";
+    str =
+        IsExtant(path_.get()) ? MayRead(path_.get()) ? "YES" : "NO" : "UNKNOWN";
     break;
   case HashInquiryKeyword("READWRITE"):
-    str = MayReadAndWrite(path_.get()) ? "YES" : "NO";
+    str = IsExtant(path_.get()) ? MayReadAndWrite(path_.get()) ? "YES" : "NO"
+                                : "UNKNOWN";
     break;
   case HashInquiryKeyword("WRITE"):
-    str = MayWrite(path_.get()) ? "YES" : "NO";
+    str = IsExtant(path_.get()) ? MayWrite(path_.get()) ? "YES" : "NO"
+                                : "UNKNOWN";
     break;
   case HashInquiryKeyword("NAME"):
     str = path_.get();
     if (!str) {
       return true; // result is undefined
     }
     break;
   }