Fix bugs found by -Wconstant-conversion improvements currently under review.

Specifically, using a an integer outside [0, 1] as a boolean constant seems to
be an easy mistake to make with things like "x == a || b" where the author
intended "x == a || x == b".

The bug caused by calling SkipUntil with three token kinds was also identified
by a VC diagnostic & reported by Francois Pichet as review feedback for my
commit r154163. I've included test cases to verify the error recovery that was
broken/poorly implemented due to this bug.

The other fix (lib/Sema/SemaExpr.cpp) seems like that code was never actually
reached in any of Clang's tests & is related to Objective C features I'm not
familiar with, so I've not been able to construct a test case for it. Perhaps
someone else can.

llvm-svn: 154325