[Clang][CMake] Use IRPGO instead of FE PGO for Cmake Caches (llvm#155957)

Currently the clang CMake caches use FE PGO for instrumentation
(LLVM_BUILD_INSTRUMENTED=ON). However, IRPGO is generally regarded as
better for performance. I am measuring about a 1.5% performance gain
when building libLLVMSupport.a using this configuration versus what
existed before this commit. I would suspect the gains are larger on
other platforms like Windows where we cannot subsume any gains using
PLO.

Author: boomanaiden154

clang/cmake/caches/PGO.cmake

@@ -5,7 +5,7 @@ set(LLVM_ENABLE_PROJECTS "clang;lld" CACHE STRING "")
 set(LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind" CACHE STRING "")
 
 set(LLVM_TARGETS_TO_BUILD Native CACHE STRING "")
-set(BOOTSTRAP_LLVM_BUILD_INSTRUMENTED ON CACHE BOOL "")
+set(BOOTSTRAP_LLVM_BUILD_INSTRUMENTED IR CACHE BOOL "")
 set(CLANG_BOOTSTRAP_TARGETS
   generate-profdata
   stage2