[overlay-syncup] sync overlay changes to Linux

[mwwa]

First cut at pulling in the recent Swift overlay changes and get them
building on Linux.

[mwwa]

@swift-ci please test

[mwwa]

@dgrove-oss Here's the syncup for my changes. The only way I can build for Linux is via the CI bot, so there'll be some back and forth spam as I fix up any build errors that I've got.

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[seabaylea]

Note that the libdispatch CI builds are currently failing with the following

/home/buildnode/jenkins/workspace/oss-swift-incremental-RA-libdispatch-linux-ubuntu-15_10/swift-corelibs-libdispatch/src/swift/Wrapper.swift:18:1: error: '@noreturn' has been removed; functions that never return should have a return type of 'Never' instead
@noreturn public func dispatchMain() {
^~~~~~~~~~
                                     -> Never

Its probably worth fixing that in this PR rather than risk having a merge conflict.

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[seabaylea]

Looks like the build was ok, but a couple of the tests failed:

FAIL: dispatch_priority
FAIL: dispatch_priority2

Those aren't related to the changes in the PR. I've also seen an intermittent failure in dispatch_priority2 and dispatch_read2 in the libdispatch CI, so it might be worth disabling the following three tests for now:

dispatch_priority
dispatch_priority2
dispatch_read2

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test

[mwwa]

@swift-ci please test