Add "ada_" prefix to enum ada_exception_catchpoint_kind

Add "ada_" prefix to enum ada_exception_catchpoint_kind

This is in preparation for making that type public, in order to be
able to use make create_ada_exception_catchpoint public as well,
making it usable from the GDB/MI implementation.

gdb/ChangeLog:

        * ada-lang.c (enum ada_exception_catchpoint_kind): Renames
        "enum exception_catchpoint_kind".  Replace the "ex_" prefix
        of all its enumerates with "ada_".  Update the rest of this
        file throughout.