Minor tweaks on doug's objc recovery patch: the caller

of isSimpleObjCMessageExpression checks the language,
so change a dynamic check into an assert.

isSimpleObjCMessageExpression is expensive, so only do it
in the common case when it is likely to matter: when the [
of the postfix expr starts on a new line.  This should avoid
doing lookahead for every array expression.

llvm-svn: 105229