Split up Parallel and LLVM'ize naming conventions.

This is one step in preparation of raising this up to
LLVM.  This hides all of the Executor stuff in a private
implementation file, leaving only the core algorithms and
the TaskGroup class exposed.  In doing so, fix up all the
variable names to conform to LLVM style.

Differential Revision: https://reviews.llvm.org/D32890

llvm-svn: 302288