Consolidate the include statements scattered across the code in accordance
with the comment in C4Include.h. The advantages are listed in the same
comment.
Furthermore, it follows llvm-include-order which is the logical
extrapolation of the project's style guideline wherever possible
(C4Include.h being the most-frequent exception).
It's still pretty useless because nobody cares in the slightest about
which extensions are available and which ones aren't, but at least now
it doesn't throw an unintelligible list of numbers at you.