[CMake] Remove {LIBCXX,LIBCXXABI,LIBUNWIND}_INSTALL_PREFIX

These variables were introduced during early work on the runtimes build
but were obsoleted by {LIBCXX,LIBCXXABI,LIBUNWIND}_INSTALL_LIBRARY_DIR.

Differential Revision: https://reviews.llvm.org/D99697

GitOrigin-RevId: 96d8c6b571e696f520a054820d13872cdc17c793
2 files changed
tree: 8b40e2bc7b07c8bae2868acd74be31d52f795f71
  1. .clang-format
  2. CMakeLists.txt
  3. LICENSE.TXT
  4. cmake/
  5. docs/
  6. include/
  7. src/
  8. test/