rustc_codegen_llvm: fix ownership of Builder.

Author: irinagpopa

src/librustc_codegen_llvm/builder.rs

@@ -27,14 +27,14 @@ use std::ptr;
 // All Builders must have an llfn associated with them
 #[must_use]
 pub struct Builder<'a, 'll: 'a, 'tcx: 'll> {
-    pub llbuilder: &'ll llvm::Builder,
+    pub llbuilder: &'ll mut llvm::Builder<'ll>,
     pub cx: &'a CodegenCx<'ll, 'tcx>,
 }
 
 impl Drop for Builder<'a, 'll, 'tcx> {
     fn drop(&mut self) {
         unsafe {
-            llvm::LLVMDisposeBuilder(self.llbuilder);
+            llvm::LLVMDisposeBuilder(&mut *(self.llbuilder as *mut _));
         }
     }
 }