Selector::getIdentifierInfoForSlot() can return NULL values, a fact

that was ignored in a few places (most notably, code
completion). Introduce Selector::getNameForSlot() for the common case
where we only care about the name. Audit all uses of
getIdentifierInfoForSlot(), switching many over to getNameForSlot(),
fixing a few crashers.

Fixed <rdar://problem/8939352>, a code-completion crasher.

llvm-svn: 125977