Update to enable compilation with latest LLVM trunk.

LLVM trunk as replaced their homebrew ```OwningPtr``` with C++11's ```std::unique_ptr```, and eliminated ```llvm/Support/system_error.h```.  This PR updates the Julia sources to match.

As an alternative to minimize preprocessor lines, I considered creating a template typedef for ```OwningPtr```.  But that seemed like it would be creating more obscure legacy code.