redecl: add test for attribute placement in function declarators

Add a new test file which demonstrates some problems which can be
seen on the git codebase. gcc does not complain about this file:

  $ gcc -Wall -c validation/function-redecl2.c
  $

... but sparse does:

  $ sparse validation/function-redecl2.c
  validation/function-redecl2.c:6:5: error: symbol 'func0' redeclared with different type (originally declared at validation/function-redecl2.c:3) - different modifiers
  validation/function-redecl2.c:13:6: error: symbol 'func1' redeclared with different type (originally declared at validation/function-redecl2.c:11) - different modifiers
  validation/function-redecl2.c:21:6: error: symbol 'func2' redeclared with different type (originally declared at validation/function-redecl2.c:18) - different modifiers
  $

Note that func0 and func2 are essentially the same example, apart from
the attribute used, to demonstrate that the issue isn't caused by the
'pure' attribute. Also, examples like func1 have occurred several times
in git and, although they can be worked around (eg. See [1]), it would
be preferable if this were not necessary.

[1] (git) commit 3d7dd2d3b6 ("usage: add NORETURN to BUG() function
definitions", 2017-05-21).

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>

validation/function-redecl2.c

@@ -0,0 +1,31 @@
+extern void exit (int __status) __attribute__ ((__noreturn__));
+
+int func0(int a) __attribute__ ((pure));
+
+__attribute__ ((pure))
+int func0(int a)
+{
+	return 0;
+}
+
+__attribute__ ((noreturn)) void func1(int a);
+
+void func1(int a)
+{
+	exit(0);
+}
+
+void func2(int a) __attribute__ ((noreturn));
+
+__attribute__ ((noreturn))
+void func2(int a)
+{
+	exit(0);
+}
+
+/*
+ * check-name: function-redecl2
+ *
+ * check-known-to-fail
+ *
+ */