[clangd] ignore parallelism level for quick tasks

This allows quick tasks without dependencies that
need to run fast to run ASAP. This is mostly useful
for code formatting.

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This fixes something that's been annoying me:
- Open your IDE workspace and its 20 open files
- Clangd spends 5 minutes parsing it all
- In the meantime you start to work
- Save a file, trigger format-on-save, which hangs because clangd is busy
- You're stuck waiting until clangd is done parsing your files before the formatting and save takes place

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D94875