[clang] Use add_llvm_install_targets

Use this function to create the install targets rather than doing so
manually, which gains us the `-stripped` install targets to perform
stripped installations.

Differential Revision: https://reviews.llvm.org/D40675

llvm-svn: 319489
GitOrigin-RevId: 669cae1f28fee3b02ca536181279cf4bbf2314d2
5 files changed