Fix a memory "leak" in VisMF's persistent streams (AMReX-Codes#3592)

## Summary

VisMF uses a std::map to keep track of the streams it opens. Each
PersistentIFStream object also has a vector used as IO buffer. When a
stream is closed, we need to free to memory used by the buffer. The
issue was it used std::vector::clear(), which only changes size()
without reducing capacity() at all. So the memory was never freed. The
issue is now fixed by std::vector::swap.

## Additional background

Thank Simon Guichandut (@simonguichandut) for reporting this! Thank Eric
Johnson (@yut23) for pinning down the location of the leak with the help
of the Massif heap profiler!

## Checklist

The proposed changes:
- [x] fix a bug or incorrect behavior in AMReX
- [ ] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX
users
- [ ] include documentation in the code and/or rst files, if appropriate

Src/Base/AMReX_VisMF.cpp

@@ -2202,7 +2202,7 @@ void VisMF::CloseStream(const std::string &fileName, bool forceClose)
     pifs.pstr = nullptr;
     pifs.isOpen = false;
   }
-  pifs.ioBuffer.clear();
+  VisMFBuffer::ClearBuffer(pifs.ioBuffer);
 }
 
 

Src/Base/AMReX_VisMFBuffer.H

@@ -29,6 +29,10 @@ public:
         ioBufferSize = iobuffersize;
     }
 
+    static void ClearBuffer (IO_Buffer& buf) {
+        IO_Buffer().swap(buf);
+    }
+
 protected:
 
     static AMREX_EXPORT Long ioBufferSize;   //!< ---- the settable buffer size