Selector: (changes made after discussing this more with Steve Naroff)

- Make Selector::getAsIdentifierInfo() private.  Using IdentifierInfo* in
  Selector is an implementation detail that clients shouldn't think about.
- Modify diagnostic emission in Sema::ProcessPropertyDecl to not use
  Selector::getAsIdentifierInfo() (which could crash when IdentifierInfo* is
  null) and instead use Selector::getAsString().
- Tidy up Selector::getAsString() implementation.

llvm-svn: 66313