[cmake] Follow LLVM's lead in creating exported tool targets for clang tools.

This is needed by downstream projects such as swift to get proper cmake
dependency information for LLVM/Clang targets.

A few months ago I added support for exporting this information for Clang
libraries. In order to be incremental, I did not add support for exporting clang
tools as well at that time. Now such support is needed, so I am committing this
incremental code.

llvm-svn: 284658
diff --git a/clang/cmake/modules/AddClang.cmake b/clang/cmake/modules/AddClang.cmake
index 6e063a7..88ee1f7 100644
--- a/clang/cmake/modules/AddClang.cmake
+++ b/clang/cmake/modules/AddClang.cmake
@@ -139,6 +139,7 @@
         -DCMAKE_INSTALL_COMPONENT=${name}
         -P "${CMAKE_BINARY_DIR}/cmake_install.cmake")
     endif()
+    set_property(GLOBAL APPEND PROPERTY CLANG_EXPORTS ${name})
   endif()
 endmacro()