Move the creation of the predefined typedef for Objective-C's 'id'

Move the creation of the predefined typedef for Objective-C's 'id'
type over into the AST context, then make that declaration a
predefined declaration in the AST format. This ensures that different
AST files will at least agree on the (global) declaration ID for 'id',
and eliminates one of the "special" types in the AST file format.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137429 91177308-0d34-0410-b5e6-96231b3b80d8