Document source_filename in LangRef.

Summary: Add documentation for new LLVM IR source_filename identifier.

Reviewers: joker.eph, majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18857

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267150 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed