[llvm] [openmp] [offload][openmp] - Remove standalone build in favor of 'runtimes' (PR #170693)

[via llvm-commits]

estewart08 wrote:

> How is it supposed to work? FWICS `llvm_gtest` target requires `find_package(LLVM)` to succeed but the runtimes CMakeLists has only:
> 
> ```
> find_package(LLVM PATHS "${LLVM_BINARY_DIR}" NO_DEFAULT_PATH NO_CMAKE_FIND_ROOT_PATH)
> ```
> 
> which cannot succeed because `LLVM_BINARY_DIR` is empty and `NO_DEFAULT_PATH` suppresses any way to specify the correct path.

See https://github.com/llvm/llvm-project/pull/170693/files#diff-f55c23c16cfc0dfde75afe6d6fe297481ed8e9221a0041360ee14ec70030a44fR100

The new 'standalone' runtimes method would require additional cmake options, LLVM_BINARY_DIR being one of them. Maybe those should be mentioned in the commit message. I wonder if a we should add a fatal error if it is not set.

https://github.com/llvm/llvm-project/pull/170693