Properly handle typos in the conditional of ?: expressions in C.

In particular, remove the OpaqueExpr transformation from r225389 and
move the correction of the conditional from CheckConditionalOperands to
ActOnConditionalOp before the OpaqueExpr is created. This fixes the
typo correction behavior in C code that uses the GNU extension for a
binary ?: (without an expression between the "?" and the ":").

llvm-svn: 227220