Dandling memory pointers in Ada catchpoints with GDB/MI.

When using the GDB/MI commands to insert a catchpoint on a specific
Ada exception, any re-evaluation of that catchpoint (for instance
a re-evaluation performed after a shared library got mapped by the
inferior) fails. For instance, with any Ada program:

    (gdb)
    -catch-exception -e program_error
    ^done,bkptno="1",bkpt={[...]}
    (gdb)
    -exec-run
    =thread-group-started,id="i1",pid="28315"
    =thread-created,id="1",group-id="i1"
    ^running
    *running,thread-id="all"
    (gdb)
    =library-loaded,[...]
    &"warning: failed to reevaluate internal exception condition for catchpoint 1: No definition of \"exec\" in current context.\n"
    &"warning: failed to reevaluate internal exception condition for catchpoint 1: No definition of \"exec\" in current context.\n"
    [...]

The same is true if using an Ada exception catchpoint.

The problem comes from the fact that that we deallocate the strings
given as arguments to create_ada_exception_catchpoint, while the latter
just makes shallow copies of those strings, thus creating dandling
pointers.

This patch fixes the issue by passing freshly allocated strings to
create_ada_exception_catchpoint, while at the same time updating
create_ada_exception_catchpoint's documentation to make it clear
that deallocating the strings is no longer the responsibility of
the caller.

gdb/ChangeLog:

        * ada-lang.c (create_ada_exception_catchpoint): Enhance
        the documentation of fields "except_string" and "condition".
        * mi/mi-cmd-catch.c (mi_cmd_catch_assert): Reallocate
        CONDITION on the heap before passing it to
        create_ada_exception_catchpoint.
        (mi_cmd_catch_exception): Likewise for EXCEPTION_NAME and
        CONDITION.

gdb/testsuite/ChangeLog:

        * gdb.ada/mi_ex_cond: New testcase.

Tested on x86_64-linux.  The "-break-list" test FAILs without
this patch.

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