[PGO] Bump the indexed profile format version

[vsk]

Summary

Along with D39446, this will make it possible to update the stable PGO hash. No existing profiles are invalidated.

[davidxl]

include/llvm/ProfileData/InstrProf.h

862–864

This only applies to FE instrumentation hashing. Can you make it clearer?

[vsk]

• Clarify comment.

[davidxl]

lgtm

include/llvm/ProfileData/InstrProf.h

enum ProfVersion {
   // function structural hash.
   Version2 = 2,
   // Version 3 supports value profile data. The value profile data is expected
   // to follow the block counter profile data.
   Version3 = 3,
   // In this version, profile summary data \c IndexedInstrProf::Summary is
   // stored after the profile header.
   Version4 = 4,
-  // The current version is 4.
+  // In this version, the frontend PGO stable hash algorithm defaults to V2.
+  Version5 = 5,
+  // The current version is 5.
   CurrentVersion = INSTR_PROF_INDEX_VERSION
 };
 const uint64_t Version = ProfVersion::CurrentVersion;
 
 const HashT HashType = HashT::MD5;
 
 inline uint64_t ComputeHash(StringRef K) { return ComputeHash(HashType, K); }
 

include/llvm/ProfileData/InstrProfData.inc

#define INSTR_PROF_RAW_MAGIC_64 (uint64_t)255 << 56 | (uint64_t)'l' << 48 | \
         (uint64_t)'f' << 16 | (uint64_t)'r' << 8 | (uint64_t)129
 #define INSTR_PROF_RAW_MAGIC_32 (uint64_t)255 << 56 | (uint64_t)'l' << 48 | \
        (uint64_t)'p' << 40 | (uint64_t)'r' << 32 | (uint64_t)'o' << 24 |  \
         (uint64_t)'f' << 16 | (uint64_t)'R' << 8 | (uint64_t)129
 
 /* Raw profile format version (start from 1). */
 #define INSTR_PROF_RAW_VERSION 4
 /* Indexed profile format version (start from 1). */
-#define INSTR_PROF_INDEX_VERSION 4
+#define INSTR_PROF_INDEX_VERSION 5
 /* Coverage mapping format vresion (start from 0). */
 #define INSTR_PROF_COVMAP_VERSION 2
 
 /* Profile version is always of type uint64_t. Reserve the upper 8 bits in the
  * version for other variants of profile. We set the lowest bit of the upper 8
  * bits (i.e. bit 56) to 1 to indicate if this is an IR-level instrumentaiton
  * generated profile, and 0 if this is a Clang FE generated profile.
  */