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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@105229 91177308-0d34-0410-b5e6-96231b3b80d8