[CMake] LLDB.framework tools handling

Summary:
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.

Reviewers: xiaobai, compnerd, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: clayborg, friss, ki.stfu, mgorny, lldb-commits, labath, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D62472

llvm-svn: 361946