[ProfileData] Refactor VTableNamePtr and CompressedVTableNamesLen (NFC) (#94859)

VTableNamePtr and CompressedVTableNamesLen are always used together to
create a StringRef in getSymtab.

We can create the StringRef ahead of time in readHeader.  This way,
IndexedInstrProfReader becomes a tiny bit simpler with fewer member
variables.  Also, StringRef default-constructs itself with its Data
and Length set to nullptr and 0, respectively, which is exactly what
we need.