[EditLine] Rewrite GetHistoryFilePath
Rewrite the GetHistoryFilePath implementation without relying on
FileSpec in the spirit of our discussion in D61994.
It changes LLDBs behavior in two ways:
- We now only use the -widehistory suffix when LLDB is built with wchar support, instead of as the fallback from when the ~/.lldb directory isn't writable.
- When the ~/.lldb directory isn't writable, we don't write any history files at all. Previously we would write them to the user's home directory (with the incorrect wide suffix), polluting ~ with a different file for every IO handler.
Differential revision: https://reviews.llvm.org/D62216