Fix build with LLVM top-of-tree.

The DIBuilder::getCU() method has been removed; we now just store the
compilation unit returned when we call DIBuilder::createCompileUnit.

module.h

@@ -41,6 +41,9 @@
 
 #include "ispc.h"
 #include "ast.h"
+#if !defined(LLVM_3_1) && !defined(LLVM_3_2) && !defined(LLVM_3_3)
+  #include <llvm/DebugInfo.h>
+#endif
 
 namespace llvm
 {
@@ -153,6 +156,10 @@ public:
     /** The diBuilder manages generating debugging information */
     llvm::DIBuilder *diBuilder;
 
+#if !defined(LLVM_3_1) && !defined(LLVM_3_2) && !defined(LLVM_3_3)
+    llvm::DICompileUnit diCompileUnit;
+#endif // LLVM_3_4+
+
 private:
     const char *filename;
     AST *ast;