Add support for ObjC keyword selectors.

- Add SelectorInfo/SelectorTable classes, modeled after IdentifierInfo/IdentifierTable.
- Add SelectorTable instance to ASTContext, created lazily through ASTContext::getSelectorInfo().
- Add SelectorInfo slot to ObjcMethodDecl.
- Add helper function to derive a SelectorInfo from ObjcKeywordInfo.

Misc: Got the Decl stats stuff up and running again...it was missing support for ObjC AST's.
llvm-svn: 42023