Fix some Wundef warnings in Compiler.h

[svenvh]

Summary

Check for definedness of the __cpp_sized_deallocation and
__cpp_aligned_new feature test macros. These will not be defined
when the feature is not available, and that prevents any code that
includes this header from compiling with -Wundef -Werror.

[joey]

LGTM!

llvm/trunk/include/llvm/Support/Compiler.h

 /// handle even over-aligned allocations.
 ///
 /// However, this doesn't make any attempt to leverage the fancier techniques
 /// like posix_memalign due to portability. It is mostly intended to allow
 /// compatibility with platforms that, after aligned allocation was added, use
 /// reduced default alignment.
 inline void *allocate_buffer(size_t Size, size_t Alignment) {
   return ::operator new(Size
-#if __cpp_aligned_new
+#ifdef __cpp_aligned_new
                         ,
                         std::align_val_t(Alignment)
 #endif
   );
 }
 
 /// Deallocate a buffer of memory with the given size and alignment.
 ///
 /// If supported, this will used the sized delete operator. Also if supported,
 /// this will pass the alignment to the delete operator.
 ///
 /// The pointer must have been allocated with the corresponding new operator,
 /// most likely using the above helper.
 inline void deallocate_buffer(void *Ptr, size_t Size, size_t Alignment) {
   ::operator delete(Ptr
-#if __cpp_sized_deallocation
+#ifdef __cpp_sized_deallocation
                     ,
                     Size
 #endif
-#if __cpp_aligned_new
+#ifdef __cpp_aligned_new
                     ,
                     std::align_val_t(Alignment)
 #endif
   );
 }
 
 } // End namespace llvm
 
 #endif