Make sure macros in the include subdirectory are not used without being defined.

Rationale: For each preprocessor macro, either the definedness is what's
meaningful, or the value is what's meaningful, or both. If definedness is
meaningful, we should use #ifdef. If the value is meaningful, we should use
and #ifdef interchangeably for the same macro, seems ugly to me, even if
undefined macros are zero if used.

This also has the benefit that including an LLVM header doesn't prevent
you from compiling with -Wundef -Werror.

Patch by John Garvin!
<rdar://problem/12189979>

llvm-svn: 163148