[DEBUGINFO] Do not output labels for empty macinfo sections.

Summary:
If there is no debug info for macros, do not emit labels for empty
macinfo sections.

Reviewers: probinson, echristo

Subscribers: aprantl, llvm-commits, JDevlieghere

Differential Revision: https://reviews.llvm.org/D43589

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325803 91177308-0d34-0410-b5e6-96231b3b80d8