libclc sequential build issue addressed in commit 0c21d6b4c8ad is
specific to cmake MSVC generator. Therefore, this PR avoids creating a
large number of targets when a non-MSVC generator is used, such as the
Ninja generator, which is used in pre-merge CI on Windows in
llvm-project repo. We plan to migrate from MSVC generator to Ninja
generator in our downstream CI to fix flaky cmake bug `Cannot restore
timestamp`, which might be related to the large number of targets.
e6d095e89c