replace for-loop based initialization

[rauhul]

replace structures like:

for i in 0..<len {
    buf.advanced(by: i).initialize(to: 0)
}

with:

buf.initialize(repeating: 0, count: len)

should bring small performance improvements

[pushkarnk]

@swift-ci please test

[millenomi]

@swift-ci please test and merge

[rudkx]

It looks like this commit might have broken the Ubuntu 18.04 test runs: https://ci.swift.org/job/oss-swift-incremental-RA-linux-ubuntu-18_04/329/console

[millenomi]

@rudkx It is not clear to me that it has — the log you linked now 404s, and while I see that the build is broken, it is broken in URLSession tests, not NSURL creation. What are you seeing?

[rudkx]

@millenomi I believe it was the same issue as the latest build:

 Fatal error: Unexpectedly found nil while unwrapping an Optional value: file TestFoundation/TestURLSession.swift, line 283

I may have jumped the gun assuming it was related to this commit, but I believe when I looked at the log earlier there were only two commits in the first build that broke, and the other seemed completely unrelated.

@shahmishal mentioned a little while ago that he is working on getting this bot blue, so perhaps it's completely unrelated to source changes and something about the configuration.