Start implementing the PTH IdentifierInfo-saving trick in PCH,

allocating IdentifierInfos with a pointer into the string data stored
in the PCH file rather than having an entry in the identifier table's
string map. However, we don't actually get these savings at the
moment, because we go through the IdentifierTable when loading
identifiers from the on-disk hash table.

This commit is for record-keeping purposes only. I'll be reverting
this change (and the PCH layout tweak that preceded it) because it
appears that implementing this optimization will collide with another,
future optimization to reduce the size of the on-disk hash table for
identifiers. That optimization is likely to provide more benefit (with
less voodoo).

llvm-svn: 70070