[flang][runtime] Flush output before INQUIRE(..., SIZE=) (#75379)

Ensure that any buffered data has tranferred to an external unit before
measuring its file size.

Author: klausler

flang/runtime/io-stmt.cpp

@@ -1219,6 +1219,7 @@ bool InquireUnitState::Inquire(
   case HashInquiryKeyword("SIZE"):
     result = -1;
     if (unit().IsConnected()) {
+      unit().FlushOutput(*this);
       if (auto size{unit().knownSize()}) {
         result = *size;
       }