[libunwind][cmake] Create `LIBUNWIND_INSTALL_INCLUDE_DIR` CACHE PATH

This is created on analogy with the other CACHE PATHs in this package,
and other `*_INSTALL_INCLUDE_DIR` in other packages.

The branching is adjusted to deduplicate some existing code, and
likewise avoid having to define this new variable more than once.

This will be used for D99484.

Reviewed By: #libunwind, compnerd

Differential Revision: https://reviews.llvm.org/D116873