[CMake] Fix installation without CLANG_BUILD_EXAMPLES

When CLANG_BUILD_EXAMPLES is not on we set the EXCLUDE_FROM_ALL
directory property for the examples/ directory to tell CMake not to
build them by default.  The AddLLVM.cmake APIs are not aware of this and
try to install targets that are not built.  This does not cause an
install-time error because CMake excludes the directory from the default
installation.  However, now that installation attaches targets to the
LLVMExports export set CMake-based applications that find_package(LLVM)
fail because the example plugin binary is not available.

Tell the AddLLVM.cmake APIs to exclude the examples from installation by
setting the EXCLUDE_FROM_ALL variable they check.

Patch by Brad King!

llvm-svn: 201799