Modified the Objective-C runtime to fetch Objective-C
isas and corresponding names from the underlying process
in a manner much quicker than the current approach.

The current approach accesses memory in the underlying
process with a random-access pattern as it walks across
the data structures associated with each isa.  This
involves a great deal of back-and-forth with debugserver,
resulting in performance problems, especially with iOS
targets.

The new approach attempts to run an expression in the
target that collects the isas and names into two large
buffers, which LLDB then collects in one shot.

<rdar://problem/12914539>