Fix recursive make in clang-tools-extra

Autoconf make (all) now properly recurses from tools/extra/Makefile into
tools/extra/test/Makefile and tools/extra/test/cpp11-migrate/Makefile. The
'all' target is responsible for creating lit config files and autogenerating
tests. Subsequent 'check-all' targets will properly work.

Re-enabling UseAuto/iterator.cpp test.

General clean-up of clang-tools-extra makefiles; removing dead targets and
removing duplicated pieces of llvm/Makefile.rules.