[memprof] Add MemProf version (#86414)

This patch adds a version field to the MemProf section of the indexed
profile format, calling the new version "version 1".  The existing
version is called "version 0".

The writer supports both versions via a command-line option:

  llvm-profdata merge --memprof-version=1 ...

The reader supports both versions by automatically detecting the
version from the header.