Move common header files into a 'support' directory; make 'testit' include -I to that directory; rename 'iterators.h' to 'iterator_test.h'; remove hard-coded paths to include files from more than 350 source files

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@171594 91177308-0d34-0410-b5e6-96231b3b80d8