[CMake] Always generate and install cmake config files on CMake>=3.0.

Currently, cmake config files are only generated and installed when CLANG_BUILD_STANDALONE set, which means config file will not be generated or installed when clang is built with llvm. This change removes that restriction.

Thanks to Don Hinton <hintonda@gmail.com>

http://reviews.llvm.org/D13453

llvm-svn: 249935