Modify the way LLDB.framework tools are collected. This allows for better fine-tuning of the install behavior downstream. Each target calls lldb_add_to_framework() individually. When entering the function, the target exists and we can tweak its very own post-build and install steps. This was not possible with the old LLDB_FRAMEWORK_TOOLS approach.
No function change otherwise.
This is a reduced follow-up from the proposal in D61952.