Export lib and exe build target names from install tree

Use the install(TARGETS) command EXPORT option for every library and
executable that we install with LLVM.  Then use the install(EXPORT)
command to provide a "LLVMExports.cmake" file that exports logical
targets for import into applications from our install 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: 201049