SwiftCompilerSources: make `fflush(stdout)` in Test.swift work on Windows

[eeckstein]

Description

It's currently disabled, because it does not compile on Windows
Therefore the swift-implemented unit tests don't work.

Also, enable public var standardError = CFileStream(fp: stderr) in Basic/Utils.swift

Reproduction

revert the "SwiftCompilerSources: exclude some low level file operations, which are not compilable on windows" commit from
#73247

Expected behavior

There should

Environment

Relevant once #73247 lands

Additional information

No response

[eeckstein]

#74766