[DWARF parser] Simplify DWARFDebugAranges generation.

There is no need to keep the whole contents of .debug_aranges section
in memory when we build address ranges table. Memory optimization that
used to be in this code (precalculate the size of vector of ranges before
filling it) is not really needed - later we will compact and resize this
vector anyway.

llvm-svn: 207457