[CMake] Use add_dependencies in add_llvm_install_targets
[CMake] Use add_dependencies in add_llvm_install_targets

Summary:
The CMake documentation says that the `DEPENDS` field of
add_custom_target is for files and output of custom commands. Adding a
dependency on a target should be done with `add_dependency`.

Differential Revision: https://reviews.llvm.org/D60879

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@359042 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm