(Ada) C++fy conditional string when catching exception.

(Ada) C++fy conditional string when catching exception.

This commit C++fy the conditional string used when catching Ada exception.

gdb/ChangeLog:

        * ada-lang.c (catch_ada_exception_command_split)
        (create_ada_exception_catchpoint) <cond_string>: Change parameter
        type.  Update code accordingly.
        (catch_ada_exception_command, catch_ada_handlers_command): Use
        C++ string instead of char* for conditional var.
        (catch_ada_assert_command_split) <cond_string>: Change parameter
        type.  Update code accordingly.
        (catch_assert_command): Use C++ string instead of char* for
        conditional var.
        * ada-lang.h (create_ada_exception_catchpoint) <cond_string>:
        Update declaration.
        * mi/mi-cmd-catch.c (mi_cmd_catch_assert, mi_cmd_catch_exception):
        Use std::string instead of char* for condition string.

Tested on x86_64-linux.