TSCBasic: fix race condition causing close errors

[Bo98]

When compiling SwiftPM (via swift-bootstrap), I was very consistently getting a "close error" that would break the compile.

This appears to be a race condition as it only happens with overlapping popen calls, likely from the subprocess in the 2nd popen call inheriting all the pipe file descriptors from the previous popen call.

What seemed to fix it for me is instead of manually closing all pipes in the subprocess, we instead set O_CLOEXEC on the pipe - this will apply to all subprocesses so concurrent popen calls are no longer a concern.

It appears this issue may have popped up before but wasn't tracked down fully: #143 (indeed stdin seemed to be the one that would always error for me), #439 and friends.

Likely fixes #344.

This issue also affects both release branches, so could also be cherry picked there (I tested 5.9 and it fixes the issue there).

[neonichu]

#439 was caused by a miscompile, but #143 may indeed be related.

[neonichu]

@swift-ci please test