Add a description to the test from r211433 explaining why it's written that way.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211465 91177308-0d34-0410-b5e6-96231b3b80d8
Introduce a Lit feature "debug_frame" and apply it to llvm/test/MC/ELF/cfi-version.ll.

.debug_frame is not emitted for targeting Windows x64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211466 91177308-0d34-0410-b5e6-96231b3b80d8