Data formatters: fix detection of C strings

Summary:
Detection of C strings does not work well for pointers. If the value object holding a (char*) pointer does not have an address (e.g., if it is a temp), the value is not considered a C string and its formatting is left to DumpDataExtractor rather than the special handling in  ValueObject::DumpPrintableRepresentation. This leads to inconsistent outputs, e.g., in escaping non-ASCII characters. See the test for an example; the second test expectation is not met (without this patch). With this patch, the C string detection only insists that the pointer value is valid. The patch makes the code consistent with how the pointer is obtained in ValueObject::ReadPointedString.

Reviewers: teemperor

Reviewed By: teemperor

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D76650