Move STLFunctionalExtras out of STLExtras

Only using that change in StringRef already decreases the number of
preoprocessed lines from 7837621 to 7776151 for LLVMSupport

Perhaps more interestingly, it shows that many files were relying on the
inclusion of StringRef.h to have the declaration from STLExtras.h. This
patch tries hard to patch relevant part of llvm-project impacted by this
hidden dependency removal.

Potential impact:
- "llvm/ADT/StringRef.h" no longer includes <memory>,
  "llvm/ADT/Optional.h" nor "llvm/ADT/STLExtras.h"

Related Discourse thread:
https://llvm.discourse.group/t/include-what-you-use-include-cleanup/5831