[Exclusivity] Treat warning violations with reabstraction thunks as e…

[devincoughlin]

…rrors

In Swift 4.1 we fixed a false negative and started looking through
reabstraction thunks to statically find exclusivity violations. To lessen
source impact, we treated these violations as warnings. This commit upgrades
those warnings to errors.

rdar://problem/34669400

[devincoughlin]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - cef3139

[atrick]

@devincoughlin It looks like this corelibs PR needs to be merged first, and is ready to merge:
swiftlang/swift-corelibs-foundation#1512

Foundation/JSONEncoder.swift:369:16: error: overlapping accesses to 'self', but modification requires exclusive access; consider copying to a local variable

[devincoughlin]

@swift-ci please test source compatibility

[devincoughlin]

I only see two violations of the new diagnostics in the compatibility suite, both in the same function in NetService's Socket+Extensions

var buffer = Data(count: Int(INET6_ADDRSTRLEN)) let ptr = buffer.withUnsafeMutableBytes { inet_ntop(AF_INET, &sin.sin_addr, $0, socklen_t(buffer.count)) }
Here an appropriate fix is to copy the count of buffer outside of the closure and pass that to inet_ntop() instead.

[devincoughlin]

And the reason the compatibility suite is passing even with this change is that NetService's 4.0 compatibility version is building in Swift 3 mode, where we still emit warnings even with this commit.

[devincoughlin]

@swift-ci please test and merge

[devincoughlin]

@swift-ci please test linux platform and merge

[devincoughlin]

@swift-ci please test and merge

[devincoughlin]

@swift-ci Please test and merge