[Support] Fix unused parameter warning in reserveExtraSpace; NFC (#106327)

Author: goldsteinn

llvm/include/llvm/Support/raw_ostream.h

@@ -157,7 +157,7 @@ class raw_ostream {
   /// So that the stream could keep at least tell() + ExtraSize bytes
   /// without re-allocations. reserveExtraSpace() does not change
   /// the size/data of the stream.
-  virtual void reserveExtraSpace(uint64_t ExtraSize) {}
+  virtual void reserveExtraSpace(uint64_t ExtraSize) { (void)ExtraSize; }
 
   /// Set the stream to be buffered, with an automatically determined buffer
   /// size.