[clang] [llvm] [CMake] Preserve clang-prebolt (PR #109351)

[Madhur Amilkanthwar via llvm-commits]

madhur13490 wrote:

Here are CMAKE variables we use downstream. 
```
-DLLVM_ENABLE_PROJECTS=bolt;clang;openmp;lld                                                                                                                                                                                                                                                                                                                                            
-DLLVM_ENABLE_RUNTIMES=compiler-rt                                                                                                                                                                                                                                                                                                                                                      
-DLLVM_TARGETS_TO_BUILD=AArch64                                                                                                                                                                                                                                                                                                                                                         
-DCLANG_DEFAULT_LINKER=lld                                                                                                                                                                                                                                                                                                                                                              
-DCMAKE_BUILD_TYPE=Release                                                                                                                                                                                                                                                                                                                                                              
                                                                                                                                                                                                                                                                                                                                                                                        
-C ../clang/cmake/caches/BOLT-PGO.cmake                                                                                                                                                                                                                                                                                                           
-DBOOTSTRAP_LLVM_ENABLE_LLD=ON                                                                                                                                                                                                                                                                                                                                                          
-DBOOTSTRAP_BOOTSTRAP_LLVM_ENABLE_LLD=ON                                                                                                                                                                                                                                                                                                                                                
-DBOOTSTRAP_CLANG_DEFAULT_LINKER=lld                                                                                                                                                                                                                                                                                                                                                    
-DBOOTSTRAP_BOOTSTRAP_CLANG_DEFAULT_LINKER=lld                                                                                                                                                                                                                                                                                                                                          
-DPGO_INSTRUMENT_LTO=Thin                                                                                                                                                                                                                                                                                                                                                               
-DCLANG_BOOTSTRAP_PASSTHROUGH=CMAKE_INSTALL_PREFIX;LLVM_TARGETS_TO_BUILD;CMAKE_BUILD_TYPE;      
 
```

https://github.com/llvm/llvm-project/pull/109351