DebugInfo: Assume an absence of ranges or high_pc on a CU means the CU is empty (devoid of code addresses)

GCC emitted these unconditionally on/before 4.4/March 2012
Clang emitted these unconditionally on/before 3.5/March 2014

This improves performance when parsing CUs (especially those using split
DWARF) that contain no code ranges (such as the mini CUs that may be
created by ThinLTO importing - though generally they should be/are
avoided, especially for Split DWARF because it produces a lot of very
small CUs, which don't scale well in a bunch of other ways too
(including size)).

git-svn-id: 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed