[CMake][Clang] Copy folder without permissions

Copying the folder keeps the original permissions by default. This
creates problems when the source folder is read-only, e.g. in a
packaging environment.
Then, the copied folder in the build directory is read-only as well.
Later on, with configure_file, ClangConfig.cmake is copied into that
directory (in the build tree), failing when the directory is read-only.

Fix that problem by copying the folder without keeping the original
permissions.

Differential Revision: https://reviews.llvm.org/D130254