[lldb] Minor cleanups at callsites of FileSpec::GetFileNameExtension

FileSpec::GetFileNameExtension returns a StringRef. In some cases we
are calling it and then storing the result in a local. To prevent
cases where we store the StringRef, mutate the Filespec, and then try to
use the stored StringRef afterwards, I've audited the callsites and made
adjustments to mitigate: Either marking the FileSpec it comes from as
const (to avoid mutations) or by not storing the StringRef in a local if
it makes sense not to.

Differential Revision: https://reviews.llvm.org/D149671