MCDwarf: Initialize MCLineTableHeader::Label

This sometimes remains null into MCLineTableHeader::Emit where we
conditionally construct a label if one isn't provided for us. We need it
to remain null (rather than just always constructing the label) so we
can identify unused line tables... which is a bit weird and maybe we can
do away with that logic one day (& on that day we can always construct
the label up-front and just have compilation units query the line table
for its label, etc)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203967 91177308-0d34-0410-b5e6-96231b3b80d8