[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358577
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