[Fuchsia] Use IR PGO in the Fuchsia toolchain build

After #155957 pointed it out, we're actually using the frontend PGO in
the Fuchsia cmake cache files.

Author: ilovepi

clang/cmake/caches/Fuchsia.cmake

@@ -179,7 +179,7 @@ set(BOOTSTRAP_LLVM_ENABLE_LLD ON CACHE BOOL "")
 set(BOOTSTRAP_LLVM_ENABLE_LTO ON CACHE BOOL "")
 
 if(FUCHSIA_ENABLE_PGO)
-  set(BOOTSTRAP_LLVM_BUILD_INSTRUMENTED ON CACHE BOOL "")
+  set(BOOTSTRAP_LLVM_BUILD_INSTRUMENTED IR CACHE BOOL "")
 
   set(_FUCHSIA_BOOTSTRAP_TARGETS
     generate-profdata