Teach ASTReader how to read only the Preprocessor state from an AST file, not the...

Teach ASTReader how to read only the Preprocessor state from an AST file, not the ASTContext state.

We use this when running a preprocessor-only action on an AST file in order to
avoid paying the runtime cost of loading the extra information.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@306760 91177308-0d34-0410-b5e6-96231b3b80d8