Fix ambiguous operator overload in libc++19

Summary:
libc++ 19 flags this statement as ambiguous.
```
error: use of overloaded operator '=' is ambiguous (with operand types 'std::pair<uint8_t *, uint8_t *>' (aka 'pair<unsigned char *, unsigned char *>') and 'void')
   76 |       other.cachedRange = {};
```

Explicitly qualify this with `std::pair<uint8_t *, uint8_t *>` to avoid the ambiguity.

Reviewed By: yozhu

Differential Revision: D64374395

fbshipit-source-id: 2bdb0e55c82b58652be54b2cca8e1dfec8dba07e

folly/io/IOBufQueue.h

@@ -73,7 +73,7 @@ class IOBufQueue {
 
     WritableRangeCacheData(WritableRangeCacheData&& other) noexcept
         : cachedRange(other.cachedRange), attached(other.attached) {
-      other.cachedRange = {};
+      other.cachedRange = std::pair<uint8_t*, uint8_t*>{};
       other.attached = {};
     }
     WritableRangeCacheData& operator=(WritableRangeCacheData&& other) noexcept {