[cmake] Fix -stripped for umbrella library install targets

Make the install-llvm-libraries-stripped and install-clang-libraries-stripped
targets depend on the individual library stripped install targets, so
that they actually install the libraries.
diff --git a/clang/CMakeLists.txt b/clang/CMakeLists.txt
index dc1413f..db82d87 100644
--- a/clang/CMakeLists.txt
+++ b/clang/CMakeLists.txt
@@ -588,6 +588,7 @@
     add_dependencies(clang-libraries ${lib})
     if(NOT LLVM_ENABLE_IDE)
       add_dependencies(install-clang-libraries install-${lib})
+      add_dependencies(install-clang-libraries-stripped install-${lib}-stripped)
     endif()
   endforeach()
 endif()