[Fuchsia] Use IR PGO in the Fuchsia toolchain build (#156060)

After #155957 pointed it out, we're actually using the frontend PGO in
the Fuchsia cmake cache files.
diff --git a/clang/cmake/caches/Fuchsia.cmake b/clang/cmake/caches/Fuchsia.cmake
index a3f86f6..46ae7c6 100644
--- a/clang/cmake/caches/Fuchsia.cmake
+++ b/clang/cmake/caches/Fuchsia.cmake
@@ -179,7 +179,7 @@
 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