Rework a bit Ada exception catchpoint support (in prep for GDB/MI)

Rework a bit Ada exception catchpoint support (in prep for GDB/MI)

This patch reworks a bit how the different steps required to insert
an Ada exception catchpoints are organized. They used to be:

  1. Call a "decode" function which does:
        1.a.  Parse the command and its arguments
        1.b.  Create a SAL & OPS from some of those arguments

  2. Call create_ada_exception_catchpoint using SAL as well
     as some of the arguments extracted above.

The bulk of the change consists in integrating step (1.b) into
step (2) in order to turn create_ada_exception_catchpoint into
a function whose arguments are all user-level concepts. This
paves the way from a straightforward implementation of the equivalent
commands in the GDB/MI interpreter.

gdb/ChangeLog:

        * ada-lang.c (ada_decode_exception_location): Delete.
        (create_ada_exception_catchpoint): Remove arguments "sal",
        "addr_string" and "ops".  Add argument "ex_kind" instead.
        Adjust implementation accordingly, calling ada_exception_sal
        to get the entities it no longer gets passed as arguments.
        Document the function's arguments.
        (catch_ada_exception_command): Use catch_ada_exception_command_split
        instead of ada_decode_exception_location, and update call to
        create_ada_exception_catchpoint.
        (catch_ada_assert_command_split): Renames
        ada_decode_assert_location.  Remove parameters "addr_string" and
        "ops", and now returns void.  Adjust implementation accordingly.
        Update the function documentation.
        (catch_assert_command): Use catch_ada_assert_command_split
        instead of ada_decode_assert_location.  Update call to
        create_ada_exception_catchpoint.