[CN] Warn or error on string literals

[dc-mak]

String literals are elaborated into a CreateReadOnly and this is currently not supported, so CN should warn or error on this (or implement it).

char f() {
  char* str = "hello";
  /*@ extract Owned<char>, 2u64; @*/
  return str[2];
}

[peterohanley]

That example seems to work now but this one does not:

char g(char *s);
/*@ spec g(pointer s);
    requires take mi = Owned<char>(s);
    ensures take mo = Owned<char>(s);
@*/
char f() {
    /*@ extract Owned<char>, 0u64; @*/
    return g("hello");
}

% cn verify issue_309.c
issue_309.c:8:9: warning: extract: index added, no resources (yet) extracted.
    /*@ extract Owned<char>, 0u64; @*/
        ^~~~~~~~~~~~~~~~~~~~~~~~~~ 
[1/1]: f -- fail
issue_309.c:9:8: error: Missing resource for calling function g
return g("hello");
       ^~~~~~~~~~ 
Resource needed: Owned<char>(a_504)
      issue_309.c:4:19: (arg mi)

a_504 is the name of the allocation containing "hello" but this name is inaccessible to the program.