[CMake] Disable BOLT instrumentation of Clang on instrumented build

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
```

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D136023

clang/CMakeLists.txt

@@ -868,7 +868,7 @@ if (CLANG_ENABLE_BOOTSTRAP)
   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)

clang/cmake/caches/BOLT-PGO.cmake

@@ -0,0 +1,11 @@
+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)