c: Add more C2y tests of initializer constraints

c: Add more C2y tests of initializer constraints

Add further tests of C2y requirements on initializers that became
constraints (so requiring diagnostics rather than violations being
undefined behavior) in N3346.

(Some of the tests for invalid string initializers might not cover all
theoretically possible cases, e.g. where char is 64-bit, but should
work on all currently supported targets.)

Tested for x86_64-pc-linux-gnu.

	* gcc.dg/c2y-init-2.c, gcc.dg/c2y-init-3.c: New tests.