When output range is only one entry, and input is low_pc/high_pc do not convert
to ranges. This helps with size of .debug_ranges/.debug_rnglists. It also helps
when either low_pc/high_pc is 0. We not generating potentially invalid ranges
that result in LLDB error.
Also fixed handling of DW_AT_subprogram with ranges. This can be created with
-fbasic-block-sections=all.
Reviewed By: maksfb
Differential Revision: https://reviews.llvm.org/D156374
75f770a68f