Found six (nmostly) identical files named 'test_allocator.h' in the libcxx test suite. Moved one to /support, made it a superset, and removed all but one of the others, and iupdated all the includes. Left the odd one (thread/futures/test_allocator.h) for later.

llvm-svn: 196174
diff --git a/libcxx/test/strings/basic.string/string.cons/alloc.pass.cpp b/libcxx/test/strings/basic.string/string.cons/alloc.pass.cpp
index b15b569..512d118 100644
--- a/libcxx/test/strings/basic.string/string.cons/alloc.pass.cpp
+++ b/libcxx/test/strings/basic.string/string.cons/alloc.pass.cpp
@@ -14,7 +14,7 @@
 #include <string>
 #include <cassert>
 
-#include "../test_allocator.h"
+#include "test_allocator.h"
 #include "min_allocator.h"
 
 template <class S>