[llvm] [pstl] Remove PSTL from the top-level monorepo (PR #143717)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-arm64-windows-msvc` running on `linaro-armv8-windows-msvc-04` while building `llvm,pstl` at step 3 "clean-build-dir".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/161/builds/6711

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 3 (clean-build-dir) failure: Delete failed. (failure)
Step 5 (build-unified-tree) failure: build (failure)
...
148.871 [1719/10/3590] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/GetDylibInterface.cpp.obj
148.878 [1718/10/3591] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/IRTransformLayer.cpp.obj
148.905 [1717/10/3592] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/IRCompileLayer.cpp.obj
148.930 [1716/10/3593] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/IndirectionUtils.cpp.obj
148.988 [1715/10/3594] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/IRPartitionLayer.cpp.obj
149.029 [1714/10/3595] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/JITTargetMachineBuilder.cpp.obj
149.052 [1713/10/3596] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/JITLinkReentryTrampolines.cpp.obj
149.070 [1712/10/3597] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/LazyObjectLinkingLayer.cpp.obj
149.079 [1711/10/3598] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/LazyReexports.cpp.obj
149.086 [1710/10/3599] Linking CXX executable bin\clang.exe
FAILED: bin/clang.exe 
C:\WINDOWS\system32\cmd.exe /C "cd . && C:\Users\tcwg\scoop\apps\cmake\3.31.6\bin\cmake.exe -E vs_link_exe --msvc-ver=1943 --intdir=tools\clang\tools\driver\CMakeFiles\clang.dir --rc=C:\PROGRA~2\WI3CF2~1\10\bin\100226~1.0\arm64\rc.exe --mt=C:\Users\tcwg\scoop\apps\llvm-arm64\current\bin\llvm-mt.exe --manifests  -- C:\Users\tcwg\scoop\apps\llvm-arm64\current\bin\lld-link.exe /nologo @CMakeFiles\clang.rsp  /out:bin\clang.exe /implib:lib\clang.lib /pdb:bin\clang.pdb /version:0.0 /machine:ARM64 /STACK:10000000 /INCREMENTAL:NO /subsystem:console && C:\WINDOWS\system32\cmd.exe /C "cd /D C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\build\tools\clang\tools\driver && C:\Users\tcwg\scoop\apps\cmake\3.31.6\bin\cmake.exe -E copy C:/Users/tcwg/llvm-worker/clang-arm64-windows-msvc/build/bin/clang.exe C:/Users/tcwg/llvm-worker/clang-arm64-windows-msvc/build/./bin/clang++.exe && cd /D C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\build\tools\clang\tools\driver && C:\Users\tcwg\scoop\apps\cmake\3.31.6\bin\cmake.exe -E copy C:/Users/tcwg/llvm-worker/clang-arm64-windows-msvc/build/bin/clang.exe C:/Users/tcwg/llvm-worker/clang-arm64-windows-msvc/build/./bin/clang-cl.exe && cd /D C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\build\tools\clang\tools\driver && C:\Users\tcwg\scoop\apps\cmake\3.31.6\bin\cmake.exe -E copy C:/Users/tcwg/llvm-worker/clang-arm64-windows-msvc/build/bin/clang.exe C:/Users/tcwg/llvm-worker/clang-arm64-windows-msvc/build/./bin/clang-cpp.exe""
LINK: command "C:\Users\tcwg\scoop\apps\llvm-arm64\current\bin\lld-link.exe /nologo @CMakeFiles\clang.rsp /out:bin\clang.exe /implib:lib\clang.lib /pdb:bin\clang.pdb /version:0.0 /machine:ARM64 /STACK:10000000 /INCREMENTAL:NO /subsystem:console /MANIFEST:EMBED,ID=1" failed (exit code 1) with the following output:
lld-link: error: failed to write output 'bin\clang.exe': permission denied

149.087 [1710/9/3600] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/Layer.cpp.obj
149.089 [1710/8/3601] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/LinkGraphLayer.cpp.obj
149.123 [1710/7/3602] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/LinkGraphLinkingLayer.cpp.obj
149.134 [1710/6/3603] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/LoadLinkableFile.cpp.obj
149.179 [1710/5/3604] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/LookupAndRecordAddrs.cpp.obj
149.218 [1710/4/3605] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/MachO.cpp.obj
149.222 [1710/3/3606] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/LLJIT.cpp.obj
149.249 [1710/2/3607] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/MachOPlatform.cpp.obj
149.328 [1710/1/3608] Linking CXX executable bin\lld.exe
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/143717