Disallow ArrayRef assignment from temporaries.

Without this, the following statements will create ArrayRefs that
refer to temporary storage that goes out of scope by the end of the
line:

  someArrayRef = getSingleElement();
  someArrayRef = {elem1, elem2};

Note that the constructor still has this problem:

  ArrayRef<Element> someArrayRef = getSingleElement();
  ArrayRef<Element> someArrayRef = {elem1, elem2};

but that's a little harder to get rid of because we want to be able to
use this in calls:

  takesArrayRef(getSingleElement());
  takesArrayRef({elem1, elem2});

Part of rdar://problem/16375365. Reviewed by Duncan Exon Smith.

llvm-svn: 283798