Make dwarf parsing multi-threaded

Loading the debug info from a large application is the slowest task
LLDB do. This CL makes most of the dwarf parsing code multi-threaded.

As a result the speed of "attach; backtrace; exit;" when the inferior
is an LLDB with full debug info increased by a factor of 2 (on my machine).

Differential revision: http://reviews.llvm.org/D13662

llvm-svn: 250821