[Build System: CMake] Build SourceKit libdispatch with just-built clang and clang++

[Rostepher]

Changed the external project build for libdispatch in tools/SourceKit to use the just-build clang and clang++ rather than the system versions. Related to #19674.

rdar://45148295

[Rostepher]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 39b2f3e1ea8b119d12c3a599c7137b17d31412a2

[compnerd]

I think that this is fine, but, it would be better to update the minimum requirements to 3.9.

[Rostepher]

@swift-ci please clean test Linux platform

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 39b2f3e1ea8b119d12c3a599c7137b17d31412a2

[Rostepher]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 39b2f3e1ea8b119d12c3a599c7137b17d31412a2

[Rostepher]

@swift-ci please test

[Rostepher]

I've run a docker build locally for Ubuntu 14.04 and it builds libdispatch without the #error unsupported compiler failure we've been seeing on in ci.swift.org.

Assuming this passes testing I'm going to go ahead and merge to unblock those bots.

[compnerd]

Ugh actually, I just realized that this breaks cross-compilation. We can no longer build a toolchain for Windows on Linux because the just built clang is for Windows, which cannot run on Linux.