[lldb][NFCI] Avoid construction of temporary std::strings in Variable

A common thing to do is to call `str().c_str()` to get a null-terminated
string out of an existing StringRef. Most of the time this is to be able
to use a printf-style format string. However, llvm::formatv can handle
StringRefs without the need for the additional allocation. Using that
makes more sense.

Differential Revision: https://reviews.llvm.org/D154890