libclang: declare blocks interfaces always

The implementation of these methods is not reliant on the availability
of the Blocks extension in the compiler.  However, when building on
Windows, the interface declaration is important for the attribution of
the DLL storage.  Without the attribution, the method implementation is
built but not made available as part of the ABI.  Use a check for the
blocks extension to determine how method signature is viewed rather than
controlling whether it is part of the interface.

clang/include/clang-c/Index.h

@@ -48,6 +48,10 @@
 #define CINDEX_VERSION_STRING                                                  \
   CINDEX_VERSION_STRINGIZE(CINDEX_VERSION_MAJOR, CINDEX_VERSION_MINOR)
 
+#ifndef __has_feature
+#define __has_feature(feature) 0
+#endif
+
 LLVM_CLANG_C_EXTERN_C_BEGIN
 
 /** \defgroup CINDEX libclang: C Interface to Clang
@@ -3856,8 +3860,6 @@ typedef enum CXChildVisitResult (*CXCursorVisitor)(CXCursor cursor,
 CINDEX_LINKAGE unsigned clang_visitChildren(CXCursor parent,
                                             CXCursorVisitor visitor,
                                             CXClientData client_data);
-#ifdef __has_feature
-#if __has_feature(blocks)
 /**
  * Visitor invoked for each cursor found by a traversal.
  *
@@ -3868,17 +3870,19 @@ CINDEX_LINKAGE unsigned clang_visitChildren(CXCursor parent,
  * The visitor should return one of the \c CXChildVisitResult values
  * to direct clang_visitChildrenWithBlock().
  */
+#if __has_feature(blocks)
 typedef enum CXChildVisitResult (^CXCursorVisitorBlock)(CXCursor cursor,
                                                         CXCursor parent);
+#else
+typedef _CXChildVisitResult *CXCursorVisitorBlock;
+#endif
 
 /**
  * Visits the children of a cursor using the specified block.  Behaves
  * identically to clang_visitChildren() in all other respects.
  */
 CINDEX_LINKAGE unsigned
 clang_visitChildrenWithBlock(CXCursor parent, CXCursorVisitorBlock block);
-#endif
-#endif
 
 /**
  * @}
@@ -5879,11 +5883,12 @@ CINDEX_LINKAGE CXResult clang_findReferencesInFile(
 CINDEX_LINKAGE CXResult clang_findIncludesInFile(
     CXTranslationUnit TU, CXFile file, CXCursorAndRangeVisitor visitor);
 
-#ifdef __has_feature
 #if __has_feature(blocks)
-
 typedef enum CXVisitorResult (^CXCursorAndRangeVisitorBlock)(CXCursor,
                                                              CXSourceRange);
+#else
+typedef struct _CXCursorAndRangeVisitorBlock *CXCursorAndRangeVisitorBlock;
+#endif
 
 CINDEX_LINKAGE
 CXResult clang_findReferencesInFileWithBlock(CXCursor, CXFile,
@@ -5893,9 +5898,6 @@ CINDEX_LINKAGE
 CXResult clang_findIncludesInFileWithBlock(CXTranslationUnit, CXFile,
                                            CXCursorAndRangeVisitorBlock);
 
-#endif
-#endif
-
 /**
  * The client's data object that is associated with a CXFile.
  */