Some fixes for PCH (de-)serialization of Objective-C AST nodes:

  - Deal with the Receiver/ClassInfo shared storage in ObjCMessageExpr
  - Implement PCH support for ImplicitParamDecl
  - Fix the handling of the body of an ObjCMethodDecl
  - Several cast -> cast_or_null fixes
  - Make Selector::getIdentifierInfoForSlot work for 1-argument, NULL
  selectors.
  - Make Selector::getAsString() work with NULL selectors.
  - Fix the names of VisitObjCAtCatchStmt and VisitObjCAtFinallyStmt
  in the PCH reader and writer; these were never getting called.

At this point, all of the pch-test tests pass for C and Objective-C.

llvm-svn: 70163