dump-c regression test of union initializers

Even in upcoming changes there must not be a situation where such
initializers cannot be dumped in a compilable fashion.

Author: tautschnig

regression/goto-instrument/dump-union/main.c

@@ -0,0 +1,8 @@
+union U {
+  int *p;
+  unsigned long long p_int;
+} u = {.p_int = 42};
+
+int main()
+{
+}

regression/goto-instrument/dump-union/test.desc

@@ -0,0 +1,12 @@
+CORE
+main.c
+--dump-c
+=(\(signed int \*\))?42
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+irep
+--
+This test must pass compiling the output generated using dump-c, which implies
+that no irep strings can occur.