Add Dispatch overlay to test configuration

[seabaylea]

When merging the NSURLSession code into Foundation, we hit the following issue in building the XCTest tests:

1.  While type-checking declaration 0x58c5ed8 at /home/baileyc/swift-corelibs-xctest/Tests/Functional/ListTests/main.swift:30:5
2.  While type-checking expression at [/home/baileyc/swift-corelibs-xctest/Tests/Functional/ListTests/main.swift:30:24 - line:30:128] RangeText="try! JSONSerialization.jsonObject(with: Data(contentsOf: URL(fileURLWithPath: CommandLine.arguments[2])))"
3.  While loading members for declaration 0x5d052d0 at <invalid loc>
4.  While deserializing 'init' (ConstructorDecl #11177) 
5.  While deserializing decl #27528 (PARAM_DECL)
6.  While deserializing decl #34459 (XREF)
7.  Cross-reference to module 'Dispatch'
    ... DispatchData

This is caused because the new Swift 3.0 overly isn't available to the XCTest tests at build/test time.

This PR adds the location for the build-time Dispatch overlay into the include path.

[modocache]

Cool, looks good to me!

We've been seeing some Linux CI failures in swift-corelibs-foundation; see #166 for details. Since the Foundation tests run before the XCTest ones, we may not be able to get any information from this CI job. I'll test locally as well.

@swift-ci please test

[modocache]

Perhaps for a future commit: I think it's strange to have build-script.py take a --libdispatch-build-dir argument, then append src/.libs to that argument when setting the LIBDISPATCH_BUILD_DIR environment variable. Should the --libdispatch-build-dir argument be renamed?

[modocache]

I ran this locally and confirmed:

1. This patch works on Ubuntu 15.
2. The crash in the Foundation tests that @briancroom mentions in Add @discardableResult annotation to applicable public API methods. #166 occurs on my machine as well.

I'll merge once the OS X CI job passes.

[modocache]

OS X build passed, so I'm going to go ahead and merge this. Thanks for chiming in on the Foundation failures discussed in #166, @seabaylea! 🙇