Use exact vector capacities to store DWARF line tables

[sque]

Summary

A large binary's DWARF info can contain line tables with thousands of entries. Since dynamically resized vectors don't have exact capacities, there could be a large amount of wasted space. This patch changes the DWARF reader to store the line table rows in vectors with exact capacities.

[mehdi_amini]

Doesn't vector have "shrink_to_fit()" for this purpose?
Isn't there a way to reserve() appropriately initially?

Also two general issues:

1. You need to add llvm-commits as a subscriber for the patch *on creation*. So now you should close it and reopen a new one, with llvm-commits (this is because for the archive, we want the mailing list to have a proper thread with the patch from the start).

2. It is nicer when reviewing patches to have the full context (git diff -U9999, or use arc).

lib/DebugInfo/DWARF/DWARFDebugLine.cpp

     // rudimentary sequences for address ranges [0x0, 0xsomething).
   }
 
+  // There could be hundreds or thousands of Rows in the line table. The vector
+  // will likely have an internal reserved memory block that is much bigger
+  // than necessary. Copy the contents to a block of memory with the exact size
+  // (or as exact as the allocator will allow).
+  std::vector<DWARFDebugLine::Row> compactRows;
+  compactRows.reserve(Rows.size());
+  std::copy(Rows.begin(), Rows.end(), compactRows.begin());
+  Rows = std::move(compactRows);
+
   return end_offset;
 }