[llvm-dwarfdump] - Teach dwarfdump to dump gdb-index section.

[llvm-dwarfdump] - Teach dwarfdump to dump gdb-index section.

gold linker's --gdb-index option currently is able to create the .gdb_index section that allows GDB to locate and read the .dwo files as it needs them,
this helps reduce the total size of the object files processed by the linker.

More info about that:
https://gcc.gnu.org/wiki/DebugFission
https://sourceware.org/gdb/onlinedocs/gdb/Index-Section-Format.html

Patch teaches dwarfdump tool to dump this section.

Differential revision: https://reviews.llvm.org/D21503

llvm-svn: 282235