debuginfo: Use symbol difference for CU length to simplify assembly reading/editing

Mucking about simplifying a test case ( https://reviews.llvm.org/D55261 ) I stumbled across something I've hit before - that LLVM's (GCC's does too, FWIW) assembly output includes a hardcode length for a DWARF unit in its header. Instead we could emit a label difference - making the assembly easier to read/edit (though potentially at a slight (I haven't tried to observe it) performance cost of delaying/sinking the length computation into the MC layer).

Reviewers: JDevlieghere, probinson, ABataev

Differential Revision: https://reviews.llvm.org/D55281

llvm-svn: 348806