Make sure methods with no return type default to "id".

This fixes a crasher in Sema::MatchTwoMethodDeclarations(), identified by selector-overload.m (just added).

Added Action::ActOnTranslationUnitScope() and renamed Action::PopScope to ActOnPopScope.

Added a Translation Unit Scope instance variable to Sema (will be very useful to ObjC-related actions, since ObjC declarations are always file-scoped).

llvm-svn: 42817