DebugInfo: Tag units as having been indexed in GNU pubnames by using a DW_AT_GNU_pubnames of DW_FORM_flag(_present) rather than sec_offsets to the pubnames/types sections

This is consistent with GDB ToT and reduces the number of relocations in
(type and compile) units, substantially reducing relocations and debug
size in fission + type units builds.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203082 91177308-0d34-0410-b5e6-96231b3b80d8