Delete some unnecessary code

Originally we have needed this code to map the isl_id of an array to its base
pointer. However, as now the isl_id contains a reference to the array itself we
obtain the base pointer from this isl_id and we do not need to add this
information to the IDToValue map.
Trim some includes. NFC.
clang-format: Cut more clangAST dependencies.

Hopefully fixing windows builds.
Sink a #include while we're here. NFC.

Format.h doesn't use stack.