[ctx_profile] Follow the pattern elsewhere for choosing the block IDs

This was an oversight in #91859. Using the subblock ID mechanism
other places that use the bitstream APIs (e.g. `BitstreamRemarkSerializer`) use.

Author: mtrofin

llvm/include/llvm/ProfileData/PGOCtxProfWriter.h

@@ -13,14 +13,15 @@
 #ifndef LLVM_PROFILEDATA_PGOCTXPROFWRITER_H_
 #define LLVM_PROFILEDATA_PGOCTXPROFWRITER_H_
 
+#include "llvm/Bitstream/BitCodeEnums.h"
 #include "llvm/Bitstream/BitstreamWriter.h"
 #include "llvm/ProfileData/CtxInstrContextNode.h"
 
 namespace llvm {
 enum PGOCtxProfileRecords { Invalid = 0, Version, Guid, CalleeIndex, Counters };
 
 enum PGOCtxProfileBlockIDs {
-  ProfileMetadataBlockID = 100,
+  ProfileMetadataBlockID = bitc::FIRST_APPLICATION_BLOCKID,
   ContextNodeBlockID = ProfileMetadataBlockID + 1
 };