Add more C2x attributes tests.

Add more C2x attributes tests.

This patch adds more tests of C2x attributes, where I found cases that
were handled correctly by my patches but missing from the original
tests.  Tests are added for -std=c11 -pedantic handling of C2x
attribute syntax and corresponding -Wc11-c2x-compat handling; for
struct [[deprecated]]; and for the [[__fallthrough__]] spelling of
[[fallthrough]] in the case of valid fallthrough attributes.

Tested for x86_64-pc-linux-gnu.

	* gcc.dg/c11-attr-syntax-1.c, gcc.dg/c11-attr-syntax-2.c,
	gcc.dg/c11-attr-syntax-3.c, gcc.dg/c2x-attr-syntax-4.c: New tests.
	* gcc.dg/c2x-attr-deprecated-1.c: Also test struct [[deprecated]].
	* gcc.dg/c2x-attr-fallthrough-1.c: Also test [[__fallthrough__]].

From-SVN: r278418