[modules] Allow VarDecls with initializers to use special var abbrev.

Update storage sizes to fit the (past) changes in the VarDecl's data model.
Update some comments.

Patch partially reviewed by Richard Smith as part of https://reviews.llvm.org/D24508


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@283444 91177308-0d34-0410-b5e6-96231b3b80d8