[flang] Refine output file generation

This patch cleans-up the file generation code in Flang's frontend
driver. It improves the layering between
`CompilerInstance::CreateDefaultOutputFile`,
`CompilerInstance::CreateOutputFile` and their various clients.

* Rename `CreateOutputFile` as `CreateOutputFileImpl` and make it
  private. This method is an implementation detail.
* Instead of passing an `std::error_code` out parameter into
  `CreateOutputFileImpl`, have it return Expected<>. This is a bit shorter
  and idiomatic LLVM.
* Make `CreateDefaultOutputFile` (which calls `CreateOutputFileImpl`)
  issue an error when file creation fails. The error code from
  `CreateOutputFileImpl` is used to generate a meaningful diagnostic
  message.
* Remove error reporting from `PrintPreprocessedAction::ExecuteAction`.
  This is only for cases when output file generation fails. This is
  handled in `CreateDefaultOutputFile` instead (see the previous point).
* Inline `AddOutputFile` into its only caller,
  `CreateDefaultOutputFile`.
* Switch from `lvm::buffer_ostream` to `llvm::buffer_unique_ostream>`
  for non-seekable output streams. This simplifies the logic in the driver
  and was introduced for this very reason in [1]
* Moke sure that the diagnostics from the prescanner when running `-E`
  (`PrintPreprocessedAction::ExecuteAction`) are printed before the actual
  output is generated.
* Update comments, add test.

NOTE: This patch relands [2]. As suggested by Michael Kruse in the
post-commit/post-revert review, I've added the following:
```
config.errc_messages = "@LLVM_LIT_ERRC_MESSAGES@"
```
in Flang's `lit.site.cfg.py.in`. This way, `%errc_ENOENT` in
output-paths.f90 gets the correct value on Windows as well as on Linux.

[1] https://reviews.llvm.org/D93260
[2] fd21d1e1

Reviewed By: ashermancinelli

Differential Revision: https://reviews.llvm.org/D108390