[LLVM-C] Add DIFile Field Accesssors

Summary:
Add accessors for the file, directory, source file name (curiously, an `Optional` value?), of a DIFile.

This is intended to replace the LLVMValueRef-based accessors used in D52239

Reviewers: whitequark, jberdine, deadalnix

Reviewed By: whitequark, jberdine

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60489

llvm-svn: 358577