Test the literal_cstr feature with different editions

Co-authored-by: Yuri Astrakhan <[email protected]>

Author: pvdrz

bindgen-tests/tests/expectations/tests/strings_cstr2_2018.rs

@@ -0,0 +1,5 @@
+// bindgen-flags: --rust-target=1.77 --rust-edition=2018 --generate-cstr
+
+const char* MY_STRING_UTF8 = "Hello, world!";
+const char* MY_STRING_INTERIOR_NULL = "Hello,\0World!";
+const char* MY_STRING_NON_UTF8 = "ABCDE\xFF";