CMake's generated installation scripts support `CMAKE_INSTALL_DO_STRIP`
to enable stripping the installed binaries. LLVM's build system doesn't
expose this option to the `install-` targets, but it's useful in
conjunction with `install-distribution`.

Add a new function to create the install targets, which creates both the
regular install target and a second install target that strips during
installation. Change the creation of all installation targets to use
this new function. Stripping doesn't make a whole lot of sense for some
installation targets (e.g. the LLVM headers), but consistency doesn't
hurt.

I'll make other repositories (e.g. clang, compiler-rt) use this in a
follow-up, and then add an `install-distribution-stripped` target to
actually accomplish the end goal of creating a stripped distribution. I
don't want to do that step yet because the creation of that target would
depend on the presence of the `install-*-stripped` target for each
distribution component, and the distribution components from other
repositories will be missing that target right now.

Differential Revision: https://reviews.llvm.org/D40620

llvm-svn: 319480