This patch simplifies writing of standalone Clang tools. As an
example, we add clang-check, a tool that runs a syntax only frontend
action over a .cc file. When you integrate this into your favorite
editor, you get much faster feedback on your compilation errors, thus
reducing your feedback cycle especially when writing new code.

The tool depends on integration of an outstanding patch to CMake to
work which allows you to always have a current compile command
database in your cmake output directory when you set
CMAKE_EXPORT_COMPILE_COMMANDS.

llvm-svn: 130306