ada: Use semantics from the RFC for declarative items mixed with statements

We want to allow statements lists with declarations *and* an exception
handler. What follows from this is that declarations declared in the
statement list are *not* visible from the exception handler, and that
the following code:

    declare
        A : Integer := 12;
    begin
        A : Integer := 15;

        <stmts>
    exception
        when others => ...

Roughly expands to:

    declare
        A : Integer := 12;
    begin
        declare
            A : Integer := 15;
        begin
            <stmts>
    exception
        when others => ...

As such, in the code above, there is no more error triggered for
conflicting declarations of `A`.

Move "Local declarations without block" into curated extensions

Restrict legal local decls in statement lists

Only accept object declarations & renamings, as well as use clauses for

gcc/ada/ChangeLog:

	* par-ch11.adb (P_Sequence_Of_Statements): Remove Handled
	parameter. Always wrap the statements in a block when there are
	declarations in it.
	* par-ch5.adb: Adapt call to P_Sequence_Of_Statements Update
	outdated comment, remove useless `Style_Checks` pragma.
	(P_Sequence_Of_Statements): Don't emit an error in core extensions
	mode. Emit an error when a non valid declaration is parsed in
	sequence of statements.
	* par.adb: Adapt P_Sequence_Of_Statements' signature
	* doc/gnat_rm/gnat_language_extensions.rst: Adapt documentation
	now.
	* gnat_rm.texi: Regenerate.
	* gnat_ugn.texi: Regenerate.

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