Make more bad uses of fallthrough attribute into pedwarns.

Various bad uses of the [[fallthrough]] attribute are constraint
violations in C2x, so need pedwarns rather than warnings.

This patch duly turns the relevant warnings into pedwarns.  The
relevant code is not specific to C, and does not know which form the
attribute was given in ([[fallthrough]] or [[gnu::fallthrough]] or
__attribute__((fallthrough))), but as I understand it these usages are
also erroneous for C++ and it seems reasonable to give a pedwarn here
even when a form other than [[fallthrough]] is being used.

The precise meaning of the standard wording about "The next statement
that would be executed" seems a but unclear in some corner cases; the
tests added keep to cases where it is clear whether or not the next
statement executed is of the required form.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc:
	* gimplify.c (expand_FALLTHROUGH_r, expand_FALLTHROUGH): Use
	pedwarn instead of warning_at for fallthrough not preceding a case
	or default label.

gcc/c-family:
	* c-attribs.c (handle_fallthrough_attribute): Use pedwarn instead
	of warning.

gcc/testsuite:
	* gcc.dg/c2x-attr-fallthrough-6.c: New test.  Split out from
	c2x-attr-fallthrough-3.c.
	* gcc.dg/c2x-attr-fallthrough-1.c: Add more tests.
	* gcc.dg/c2x-attr-fallthrough-2.c: Update expected diagnostics.
	* gcc.dg/c2x-attr-fallthrough-3.c: Split inside-switch part of
	test out to c2x-attr-fallthrough-6.c.

From-SVN: r278599