Load exported lib and exe targets from LLVMConfig

Teach each package configuration file to load the LLVMExports file for
its corresponding tree.  This will allow application CMake code to use
logical library and executable target names from LLVM as if they were in
our own build process (e.g. LLVMSupport).  CMake will have enough
information to propagate LLVM library link dependencies automatically
while configuring applications.

Contributed by Brad King.

llvm-svn: 201051