Update .debug_line section version information to match DWARF version.

In the past the compiler always emitted .debug_line version 2, though some opcodes from DWARF 3 (e.g. DW_LNS_set_prologue_end, DW_LNS_set_epilogue_begin or DW_LNS_set_isa) and from DWARF 4 could be emitted by the compiler. 

This patch changes version information of .debug_line to exactly match the DWARF version. For .debug_line version 4, a new field maximum_operations_per_instruction is emitted. 

Differential Revision: https://reviews.llvm.org/D16697

llvm-svn: 285355