[ADT] Fix IWYU for hashing-adjacent files (#194297)
Add explicit includes so these files keep building after we trim
transitive includes from xxhash.h.
For example, FoldingSet.cpp calls llvm::uninitialized_copy, which is
declared in llvm/ADT/STLExtras.h and today reaches the file only
transitively.
Also drop the vestigial `#include "llvm/ADT/Hashing.h"` from
llvm/ADT/STLExtras.h — no name from Hashing.h is used there.
