[lldb] Change return type of FileSpec::GetFileNameExtension

These don't really need to be in ConstStrings. It's nice that comparing
ConstStrings is fast (just a pointer comparison) but the cost of
creating the ConstString usually already includes the cost of doing a
StringRef comparison anyway, so this is just extra work and extra memory
consumption for basically no benefit.

Differential Revision: https://reviews.llvm.org/D149300