[CMake] Disable BOLT instrumentation of Clang on instrumented build

[Amir]

Summary

This enables multi-stage PGO build optimized by BOLT using BOLT.cmake cache.

The issue is that -DPGO_BUILD_CONFIGURATION cache file is passed to both
stage2-instrumented and stage2-optimized builds (for them to be identical),
but in case of BOLT.cmake, it doesn't make sense to BOLT-instrument the
instrumented binary (it's not going to be optimized). Hence turn off
CLANG_BOLT_INSTRUMENT code if LLVM_BUILD_INSTRUMENTED is enabled.

The final workflow that enables multi-stage InstrPGO+ThinLTO+BOLT Clang build:

cmake <llvm-project>/llvm -GNinja -DLLVM_ENABLE_LLD=ON \
  -DBOOTSTRAP_LLVM_ENABLE_LLD=ON -DBOOTSTRAP_BOOTSTRAP_LLVM_ENABLE_LLD=ON \
  -DPGO_INSTRUMENT_LTO=Thin -C llvm-project/clang/cmake/caches/BOLT-PGO.cmake
ninja stage2-clang++-bolt

[Amir]

Add BOLT-PGO cmake cache file

clang/CMakeLists.txt

foreach(target ${CLANG_BOOTSTRAP_TARGETS})
     if(target MATCHES "^stage[0-9]*")
       add_custom_target(${target} DEPENDS ${NEXT_CLANG_STAGE}-${target})
     endif()
 
     ExternalProject_Add_StepTargets(${NEXT_CLANG_STAGE} ${target})
   endforeach()
 endif()
 
-if (CLANG_BOLT_INSTRUMENT)
+if (CLANG_BOLT_INSTRUMENT AND NOT LLVM_BUILD_INSTRUMENTED)
   set(CLANG_PATH ${LLVM_RUNTIME_OUTPUT_INTDIR}/clang)
   set(CLANGXX_PATH ${CLANG_PATH}++)
   set(CLANG_INSTRUMENTED ${CLANG_PATH}-bolt.inst)
   set(CLANGXX_INSTRUMENTED ${CLANGXX_PATH}-bolt.inst)
   set(CLANG_OPTIMIZED ${CLANG_PATH}-bolt)
   set(CLANGXX_OPTIMIZED ${CLANGXX_PATH}-bolt)
 
   # Instrument clang with BOLT

clang/cmake/caches/BOLT-PGO.cmake

+set(LLVM_ENABLE_PROJECTS "bolt;clang;lld" CACHE STRING "")
+
+set(CLANG_BOOTSTRAP_TARGETS
+  stage2-clang++-bolt
+  CACHE STRING "")
+set(BOOTSTRAP_CLANG_BOOTSTRAP_TARGETS
+  clang++-bolt
+  CACHE STRING "")
+
+set(PGO_BUILD_CONFIGURATION ${CMAKE_CURRENT_LIST_DIR}/BOLT.cmake CACHE STRING "")
+include(${CMAKE_CURRENT_LIST_DIR}/PGO.cmake)