Simplify Windows implementation of Compilation::performSimpleCommand

[hughbe]

See https://msdn.microsoft.com/en-us/library/eyw7eyfw.aspx for the docs for _putenv_s

I also get the Microsoft Code Analysis warning that

Warning C6031 Return value ignored: '_putenv'

I think that the code analysis has a point here, but I'm not sure if I've got the right fix here. Now, if we can't set the environment variable, we return the error code instead of continuing with compilation as we did before.

Perhaps there are better options

• assert that the error code is 0, as we expect this to always work
• print some kind of informative message that we failed (a warning?), and continue compilation

@jrose-apple as the code owner of swiftDriver what do you think?

[jrose-apple]

I'd like to say we should assert because it increases our chances of recovery. Maybe we can assert in debug builds and bail out early in release builds?

Thanks for fixing this, by the way. I guess none of us are Windowsy enough to know about _putenv_s.

[hughbe]

@swift-ci please smoke test

[hughbe]

@swift-ci please smoke test OS X Platform

[hughbe]

Looks like it's not impossible for setenv to return anything but 0... what do you think we should do?

[jrose-apple]

You got your assertion backwards. :-)

[hughbe]

Ahhhh facepalm

[hughbe]

@swift-ci please smoke test