[CMake] [Darwin] Add support for generating Xcode-compatible toolchains that xcodebui...

[CMake] [Darwin] Add support for generating Xcode-compatible toolchains that xcodebuild and xcrun can search

Summary:
Sometimes you want to install a custom compiler and use it like the system compiler without overriding the system compiler. This patch lets you create xctoolchains that the darwin command line tools can use.

To use this patch set LLVM_CREATE_XCODE_TOOLCHAIN=On in your CMake invocation and build the `install-code-toolchain` target.

After installation you can set the envar EXTERNAL_TOOLCHAINS_DIR to your installed Toolchains directory, and the TOOLCHAINS envar to the toolchain identifier (ex org.llvm.3.8.0svn). This will then cause /usr/bin/clang to call your newly installed clang.

Reviewers: Bigcheese, bogner

Subscribers: tobiasfar, llvm-commits

Differential Revision: http://reviews.llvm.org/D13605

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250450 91177308-0d34-0410-b5e6-96231b3b80d8