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>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163148 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm-c/Core.h

@@ -2688,7 +2688,7 @@ namespace llvm {
   
   template<typename T>
   inline T **unwrap(LLVMValueRef *Vals, unsigned Length) {
-    #if DEBUG
+    #ifdef DEBUG
     for (LLVMValueRef *I = Vals, *E = Vals + Length; I != E; ++I)
       cast<T>(*I);
     #endif

include/llvm/Support/Compiler.h

@@ -24,7 +24,7 @@
 /// does not imply the existence of any other C++ library features.
 #if (__has_feature(cxx_rvalue_references)   \
      || defined(__GXX_EXPERIMENTAL_CXX0X__) \
-     || _MSC_VER >= 1600)
+     || (defined(_MSC_VER) && _MSC_VER >= 1600))
 #define LLVM_USE_RVALUE_REFERENCES 1
 #else
 #define LLVM_USE_RVALUE_REFERENCES 0

include/llvm/Support/FileSystem.h

@@ -40,7 +40,7 @@
 #include <string>
 #include <vector>
 
-#if HAVE_SYS_STAT_H
+#ifdef HAVE_SYS_STAT_H
 #include <sys/stat.h>
 #endif
 
@@ -596,7 +596,7 @@ private:
   mapmode Mode;
   uint64_t Size;
   void *Mapping;
-#if LLVM_ON_WIN32
+#ifdef LLVM_ON_WIN32
   int FileDescriptor;
   void *FileHandle;
   void *FileMappingHandle;