Initial integration of Swift overlay into libdispatch build

[dgrove-oss]

This commit adds a src/swift directory that contains
Dispatch.swift and Dispatch.c (based on existing files in
swift/stdlib/public/SDK/Dispatch) and a module.map.in template.
We made some initial changes to Dispatch.swift and Dispatch.c to
adapt them to Linux by dropping MACH API functions/adapters.

The libdispatch build process is extended to include Dispatch.o
in libdispatch.so and to generate Dispatch.swiftmodule,
Dispatch.swiftdoc and a module.map file when configure is
given a --with-swift-sdk argument.

Swift programs that import Dispatch can be compiled, linked,
and run against the generated files, but must include some
extra command line arguments due to the use of @convention(block)
to bridge from Swift blocks to the -fblocks expected by dispatch.
In particular, the following flags are currently needed to compile
a Swift program: -Xcc -fblocks -Xlinker -lBlocksRuntime

[billabt]

In Dispatch.swift, typealias is deprecated, the new term is associatedtype. So, line 22:

public typealias dispatch_block_t = @convention(block) () -> Void

should be this instead...

public associatedtype dispatch_block_t = @convention(block) () -> Void

[ddunbar]

Do we have a plan yet for how to get the -fblocks flag added when appropriate for Swift?

For the -lBlocksRuntime argument, you may be able to deal with this by adding a link declaration to your module map:
http://clang.llvm.org/docs/Modules.html#link-declaration

[mwwa]

It's unfortunate that all the C-defines need computed variables to be shimmed over but it looks like that's also a direct mirror of the stdlib version of this code.

[mwwa]

I mirror ddunbar's suggestion about using the module link line to try including BlocksRuntime. I'm not in a position to check if that works on Linux myself though.

[dgrove-oss]

@billabt associatedtype doesn't work in this context (not sure if this is by design, or a bug). Using a swiftc I built from git as of this morning, I get:
``
/home/vagrant/test-build/../swift-corelibs-libdispatch/src/swift/Dispatch.swift:22:8:

error: associated types can only be defined in a protocol; define a type or introduce a 'typealias' to satisfy an associated type requirement
public associatedtype dispatch_block_t = @convention(block) () -> Void
^
/home/vagrant/test-build/../swift-corelibs-libdispatch/src/swift/Dispatch.swift:22:43: error: attribute can only be applied to types, not declarations
public associatedtype dispatch_block_t = @convention(block) () -> Void
^
/home/vagrant/test-build/../swift-corelibs-libdispatch/src/swift/Dispatch.swift:22:53: error: expected declaration
public associatedtype dispatch_block_t = @convention(block) () -> Void

[dgrove-oss]

Thanks @mwwa and @ddunbar for the linker suggestion. I had missed that I could have multiple linker directives in the module.map file. I had tried putting both dispatch and BlocksRuntime into a single linker directive, which didn't work. Updated module.map.in to have two linker directives, which does work.

[dgrove-oss]

Squashed changes into a single commit and updated commit comment to reflect now only needing -Xcc -fblocks.

[dgrove-oss]

Updated & squashed with a change to the command line argument of configure to --with-swift-toolchain as requested in the review of swiftlang/swift#1212

[dgrove-oss]

@MadCoder @mwwa. Any additional changes needed? Would like to get this merged so we can continue with next steps. Thanks.

[mwwa]

No I'm happy, I'll merge momentarily.