Cleanup __config indention NFC

This commit indents each level by two space characters, e.g.

#if defined(CONDITION)
#  define _LIBCPP_NAME VALUE
#else
#  define _LIBCPP_NAME VALUE
#endif

The simple #ifndef, #define, and #endif sequences are not indented, e.g.

#ifndef _LIBCPP_NAME
#define _LIBCPP_NAME ...
#endif


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@326027 91177308-0d34-0410-b5e6-96231b3b80d8