Merge "special" types from different modules in the AST reader.

Different modules may have different views of the various "special"
types in the AST, such as the redefinition type for "id". Merge those
types rather than only considering the redefinition types for the
first AST file loaded.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174234 91177308-0d34-0410-b5e6-96231b3b80d8