DebugInfo: .debug_line DWARF64 support

This adds support for the 64-bit DWARF format, but is still limited to
less than 4GB of debug data by the DataExtractor class.  Some versions
of the GNU MIPS toolchain generate 64-Bit DWARF even though it isn't
actually necessary.

Differential Revision: http://reviews.llvm.org/D1988

llvm-svn: 238434