First step of refactoring variable handling in the

expression parser.  There shouldn't be four separate
classes encapsulating a variable.

ClangExpressionVariable is now meant to be the
container for all variable information.  It has
several optional components that hold data for
different subsystems.

ClangPersistentVariable has been removed; we now
use ClangExpressionVariable instead.

llvm-svn: 111600