Use optimized tablegen in clangd builds

A recent [change](https://github.com/llvm/llvm-project/commit/50d96f59d0966bdda6b3ac4f8964ed572435b847) to llvm-tablegen made it unusable under tsan. our bot would OOM all builds ever since. this flag should enable us to build tblgen in native setup, without tsan.
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