modify Xcode build to use cmake/ninja for internal llvm/clang

This change restores the Xcode build to working after Makefile support
was stripped from LLVM and clang recently.

With this change, the Xcode build now requires cmake (2.8.12.2+).
The cmake must either be on the path that Xcode sees, or it must
exist in one of the following locations:

* /usr/local/bin/cmake
* /opt/local/bin/cmake
* $HOME/bin/cmake

If the ninja build tool is present on the path, it will be used.
If not, ninja will be cloned (via git), bootstrap-built, and
used for the llvm/clang build.

LLDB now requires a minimum deployment target of OS X 10.9.  Prior
to this, it was 10.8.  The llvm/clang cmake build will not run
with Xcode 7.2 or Xcode 7.3 beta's compiler with the minimum
deployment target set to anything lower than 10.9.  This is
related to #include <atomic>.

When llvm or clang source code does not exist in the lldb tree,
it will be cloned via git using http://llvm.org/git/{project}.git.
Previously it used SVN.  If this causes any heartache, we can
make this smarter, autodetect an embedded svn and use svn instead.
(And/or use SVN if a git command is not available).

This change also fixes an lldb-mi linkage failure (needed
libncurses) as exposed by one of the LLVM libs.

llvm-svn: 259027