[update-checkout] Add support to clone libcxx and clang-tools-extra f…

[shahmishal]

…or master, master-next and swift-5.0-branch scheme

[shahmishal]

@swift-ci smoke test

[shahmishal]

@swift-ci smoke test Linux

[compnerd]

@shahmishal - libc++ without libc++abi isn't that great of an idea. If you are going to add libc++, Id say you should add libc++abi as well.

[shahmishal]

cc/ @dexonsmith @benlangmuir

@compnerd Thanks for pointing this out to me.

[benlangmuir]

cc/ @ldionne

[dexonsmith]

@shahmishal - libc++ without libc++abi isn't that great of an idea. If you are going to add libc++, Id say you should add libc++abi as well.

On Darwin, the libc++ headers are NOT in the SDK, and live along-side Clang. They are needed in order to create a working C++ toolchain. But the libc++abi headers are in the SDK, so they aren't needed here.

Perhaps installing the libc++ headers should be conditional on the platform (i.e., only on macOS)?

[compnerd]

@shahmishal - oh if you are ensuring that libc++ is not built, then that is okay. But you can't really build libc++ without libc++abi (or libsupc++ or libcxxrt, but those paths aren't as well tested) or msvcrt.

[dexonsmith]

Right, we're just looking to install libc++ headers here. Not trying to build libc++.1.dylib.

[shahmishal]

@swift-ci smoke test Linux