[clang] [flang] [lld] [llvm] [mlir] [RFC] Use pre-compiled headers to speed up LLVM build (~1.5-2x) (PR #173868)

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sharkautarch wrote:

> FWIW, some test feedback on this patchset:
> 
> On Ubuntu 20.04, with the stock system libstdc++ 9, I get this:
> 
> ```
> In file included from <built-in>:1:
> In file included from llvm-project/llvm/build-clang/third-party/unittest/CMakeFiles/llvm_gtest.dir/cmake_pch.hxx:6:
> In file included from llvm-project/llvm/include/llvm/Support/pch.h:74:
> In file included from /usr/lib/gcc/aarch64-linux-gnu/9/../../../../include/c++/9/execution:32:
> In file included from /usr/lib/gcc/aarch64-linux-gnu/9/../../../../include/c++/9/pstl/glue_execution_defs.h:52:
> In file included from /usr/lib/gcc/aarch64-linux-gnu/9/../../../../include/c++/9/pstl/algorithm_impl.h:25:
> In file included from /usr/lib/gcc/aarch64-linux-gnu/9/../../../../include/c++/9/pstl/parallel_backend.h:14:
> /usr/lib/gcc/aarch64-linux-gnu/9/../../../../include/c++/9/pstl/parallel_backend_tbb.h:19:10: fatal error: 'tbb/blocked_range.h' file not found
>    19 | #include <tbb/blocked_range.h>
>       |          ^~~~~~~~~~~~~~~~~~~~~
> ```

I think the simplest workaround for that error w/ gcc+libstdc++9 would be to remove the `#include <execution>` line from `llvm-project/llvm/include/llvm/Support/pch.h`
Since it seems that the only place in llvm-project where the execution syshdr is included is in libcxx, so precompiling that header doesn't even help if you're not building libcxx

https://github.com/llvm/llvm-project/pull/173868