[LLVM-C] Add DIFile Field Accesssors
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
Differential Revision: https://reviews.llvm.org/D60489