[DWARFv5] llvm-mc support for new unit header.

This is for running the assembler with -g (to emit DWARF describing
the assembler source).

Differential Revision: http://reviews.llvm.org/D30475


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@296541 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed