[CMake] Give lldb tools functional install targets when building LLDB.framework

Summary:
This change makes the install targets for lldb tools functional when
building for the framework.

I am currently working on the install rules for lldb-framework and this will
let me make `install-lldb-framework` rely on `install-lldb-argdumper` for
instance. This is especially important for `install-lldb-framework-stripped`. It
is much better for `install-lldb-framework-stripped` to rely on
`install-lldb-argdumper-stripped` than to copy and strip lldb-argdumper
manually.

Differential Revision: https://reviews.llvm.org/D49038

llvm-svn: 337202