Add basic support for .debug_ranges section to LLVM's DebugInfo library.

This section (introduced in DWARF-3) is used to define instruction address
ranges for functions that are not contiguous and can't be described
by low_pc/high_pc attributes (this is the usual case for inlined subroutines).
The patch is the first step to support fetching complete inlining info from DWARF.

Reviewed by Benjamin Kramer.

llvm-svn: 162657