Disallow ArrayRef assignment from temporaries.

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