[lldb][NFC] Remove unnecessary std::string temporaries

The existing code was taking three substrings from a regex match and converting
to std::strings prior to using them. This may have been done to address
null-termination concerns, but this is not the case:

1. `name` was being used to call `c_str()` and then implicitly converted back to
a `StringRef` on the call to `ToAddress`. While the path `const char *` ->
`StringRef` requires null-termination, we can simply use the original StringRef.
2. `str_offset` was being converted back to a StringRef in order to call a
member method. Member methods can't handle non-null termination.
3. `sign` simply had it's 0-th element accessed.