Apart from storing/retrieving the previous redeclaration from PCH, also...

Apart from storing/retrieving the previous redeclaration from PCH, also store/retrieve the most recent
redeclaration. That way we are sure that the full redeclarations chain is loaded.

When using chained PCHs, first declarations point to the most recent redeclarations in the same PCH.
To address this use a REDECLS_UPDATE_LATEST record block to keep track of which first declarations need
to point to a most recent redeclaration in another PCH.

llvm-svn: 110125