Fix C++23 build error in llvm/Support/Caching.h

[1] instantiates ~std::unique_ptr<MemoryBuffer>, so it must
be a complete type.

[1] https://github.com/llvm/llvm-project/blob/4b89704504dde687ba7983e034cb246581fd1407/llvm/include/llvm/Support/Caching.h#L121

Author: Victor Hugo Vianna Silva

llvm/include/llvm/Support/Caching.h

@@ -17,11 +17,10 @@
 
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Error.h"
+#include "llvm/Support/MemoryBuffer.h"
 
 namespace llvm {
 
-class MemoryBuffer;
-
 /// This class wraps an output stream for a file. Most clients should just be
 /// able to return an instance of this base class from the stream callback, but
 /// if a client needs to perform some action after the stream is written to,