Export lib and exe build target names from build tree

Record every logical target that we install with install(TARGETS) in a
global LLVM_EXPORTS property.  Then use the export(TARGETS) command to
provide a "LLVMExports.cmake" file that exports logical targets for
import into applications directly from our build tree.

The "LLVMExports.cmake" file is not meant for direct inclusion by
application code but should be included by "LLVMConfig.cmake" in a
future change.

Contributed by Brad King.

llvm-svn: 201050