Frontend: Fix layering between create{,Default}OutputFile, NFC

Fix layering between `CompilerInstance::createDefaultOutputFile` and the
two versions of `createOutputFile`.

- Add missing configuration flags to `createDefaultOutputFile` so that
  GeneratePCHAction and GenerateModuleFromModuleMapAction can use it.
  They previously promised that temporary files were turned on; now
  `createDefaultOutputFile` handles that logic.
- Lift the logic handling `InFile` and `Extension` to
  `createDefaultOutputFile`, since it's only the callers of that
  function that are using it.
- Rename the deeper of the two `createOutputFile`s to
  `createOutputFileImpl` and make it private to `CompilerInstance` (to
  prove that no one else is using it).
- Sink the logic for adding to `CompilerInstance::OutputFiles` down to
  `createOutputFileImpl`, allowing two "optional" (but always used)
  `std::string*` out parameters to be removed.
- Instead of passing a `std::error_code` out parameter into
  `createOutputFileImpl`, have it return `Expected<>`.
- As a drive-by, inline `CompilerInstance::addOutputFile` into its only
  caller, `createOutputFileImpl`.

Clean layering makes it easier for a future commit to extract
`createOutputFileImpl` out of `CompilerInstance`.

Differential Revision: https://reviews.llvm.org/D93248