NFC. Move definition of _LIBCPP_ASSERT into __debug header and remove external include guards.

Things done in this patch:

1. Make __debug include __config since it uses macros from it.

2. The current method of defining _LIBCPP_ASSERT is prone to redefinitions. Move
the null _LIBCPP_ASSERT definition into the __debug header to prevent this.

3. Remove external <__debug> include gaurds. <__debug> guards almost all of its
contents internally. There is no reason to be doing it externally.

This patch should not change any functionality.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@215332 91177308-0d34-0410-b5e6-96231b3b80d8