di: Provide safe wrappers for LLVM types

Provide Rust wrappers for LLVM metadata and debug info types and
aim to wrap LLVM C API functions as methods of these wrappers,
therefore try to make the logic of sanitizing the DI as safe
as possible.

The wrappers are stored in two separate modules:

* `ir` - for general LLVM IR types like `Metadata` or `MDNode`.
* `di` - for types representing debug information nodes like
  `DINode`, `DIType` etc.

Author: vadorovsky

src/linker.rs

@@ -449,7 +449,7 @@ impl Linker {
         // programs and maps and remove dead code.
 
         unsafe {
-            llvm::DIFix::new(self.context, self.module).run();
+            llvm::DISanitizer::new(self.context, self.module).run();
 
             llvm::optimize(
                 self.target_machine,