This reverts commit 272d1b44efdedb68c194970a610f0ca1b7b769c5,
and the follow-up fix in d209d1340b99d4fbd325dffb5e13b757ab8264ea.
Even after the follow-up fix, building with an empty
CMAKE_INSTALL_PREFIX errors out with errors like this:
CMake Error at /b/s/w/ir/x/w/llvm-llvm-project/libcxx/modules/CMakeLists.txt:215 (file):
file RELATIVE_PATH must be passed a full path to the directory:
lib/x86_64-pc-windows-msvc
407937036f