Added the concept of a Read-Eval-Print-Loop to LLDB.

A REPL takes over the command line and typically treats input as source code.
REPLs can also do code completion.  The REPL class allows its subclasses to
implement the language-specific functionality without having to know about the
IOHandler-specific internals.

Also added a PluginManager-based way of getting to a REPL given a language and
a target.

Also brought in some utility code and expression options that are useful for
REPLs, such as line offsets for expressions, ANSI terminal coloring of errors,
and a few IOHandler convenience functions.

llvm-svn: 250753