Add support for character option types.

This will allow, in a subsequent patch, the addition of a global
setting that allows the user to specify a single character that
LLDB will recognize as an escape character when processing arg
strings to accomodate differences in Windows/non-Windows path
handling.

Differential Revision: http://reviews.llvm.org/D6887
Reviewed by: Jim Ingham

llvm-svn: 225694