Lack of support for CFString functions

[feliperodri]

CBMC version: develop
Operating system: macOS Mojave 10.14.6
Exact command line resulting in the issue: N/A.
What behaviour did you expect: Support for CFString library.
What happened instead: **** WARNING: no body for function __builtin___CFStringMakeConstantString

[tautschnig]

Would you have an example using this library, including the command-line needed to use it? Also, the example would really help to understand to what level of detail this function needs to be modelled. As far as I understand, a precise implementation would have to keep around a map to ensure that the same two strings passed as argument result in the same address pointed to by the result.