Looking at our memory usage with Instruments when debugging a large application

we say that the vectors of DWARFDebugInfoEntry objects were the highest on the
the list. 

With these changes we cut our memory usage by 40%!!! I did this by reducing
the size of the DWARFDebugInfoEntry from a previous:

uint32_t offset
uint32_t parent_idx
uint32_t sibling_idx
Abbrev * abbrev_ptr

which was 20 bytes, but rounded up to 24 bytes due to alignment. Now we have:

uint32_t offset
uint32_t parent_idx
uint32_t sibling_idx
uint32_t abbr_idx:15,       // 32767 possible abbreviation codes
         has_children:1,    // 0 = no children, 1 = has children
         tag:16;            // DW_TAG_XXX value

This gets us down to 16 bytes per DIE. I tested some VERY large DWARF files
(900MB) and found there were only ~700 unique abbreviations, so 32767 should
be enough for any sane compiler. If it isn't there are built in assertions
that will fire off and tell us.

llvm-svn: 144975