[CMake] First pass at adding support for clang bootstrap builds to CMake

[CMake] First pass at adding support for clang bootstrap builds to CMake

Summary:
This patch adds a new CLANG_ENABLE_BOOTSTRAP option to CMake which adds targets for building a stage2 bootstrap compiler. The targets are:

bootstrap-configure
bootstrap-build
bootstrap (same as bootstrap-configure and bootstrap-build)
bootstrap-install
bootstrap-check-llvm
bootstrap-check-clang
bootstrap-check-all

If you are using 3.3.20150708 or greater it utilizes the ninja USES_TERMINAL_* settings on the external project so that the output is properly buffered.

Reviewers: bogner, chandlerc

Subscribers: filcab, cfe-commits

Differential Revision: http://reviews.llvm.org/D11743

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@244070 91177308-0d34-0410-b5e6-96231b3b80d8