Add -enable-cxx-interop flag and support for extern "C" {}

[pschuh]

This is an attempt to get the basic c++ interop working in a flag protected fashion.

Here a forum post for more context: https://forums.swift.org/t/c-interop/25567.

Some context on the changes:
The existing swift header files assume that c means that it is the swift compiler and c++ means that it is just clang. This changes them to also use the swift to properly support inclusion from swift in c++ mode. There will probably be other libraries which need support to compile in c++ mode, but this is enough to make the basic tests pass.
Also, support for extern "C" {} is needed to get anywhere with the current headers, so add that as well.

[pschuh]

@swift-ci please test

[pschuh]

Exposed only as a frontend flag.

[jrose-apple]

Looks pretty solid for an initial start! There's one serious comment in here (in SwiftStddef.h) and then a bunch of small things.

[pschuh]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 098cac2d6bc45997f3acdea50380de4d5c18cd29

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 098cac2d6bc45997f3acdea50380de4d5c18cd29

[pschuh]

@swift-ci please test linux

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - fa69a73

[pschuh]

@swift-ci please test linux

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - fa69a73

[pschuh]

Hmm. Looks like Interpreter/withoutActuallyEscaping.swift is randomly failing quite a bit now.

[jrose-apple]

@atrick, did something get messed up on Linux? Or recently fixed on Linux?

[atrick]

The withoutActuallyAliasing test was failing when optimization runs (even though it's an Interpreter test, apparently it runs with optimizations sometimes).
For some reason, we only run the tests with optimization on Linux, not mac:
"check-swift-all-optimize-linux-x86_64"

[atrick]

@swift-ci test linux

[jrose-apple]

It's a supported configuration to run the JIT with optimizations.