Without this, GCH serialization of late template expansions will
encounter an assertion failure.
Changes were required to ASTReader::ReadString to add support for
reading a string from a RecordDataImpl (which is the type consumed by
ReadToken) rather than a RecordData.
#pragma pack slot names are read into preprocessor storage.
This requires incidental changes [Parse,Sema] to move PragmaPackInfo to Sema
There is precedent for ASTReader/ASTWriter to poke into Sema.