Disallow assignment to rvalue Range objects (and StringPiece in particular)

Disallow assignment to rvalue Range objects (and StringPiece in particular)

Summary:
Before this diff the code below compiled and did absolutely nothing of interest:

StringPiece fun();
...
fun() = "hello";

i.e. assignment to an rvalue range was allowed. Such code is almost always, if not always, in error. This diff fixes that.

Test Plan: ran unittests

Reviewed By: ldbrandy@fb.com

Subscribers: mpawlowski, net-systems@, folly-diffs@, yfeldblum

FB internal diff: D1825360

Signature: t1:1825360:1423097817:fdaaf893cd1abbe71dc857a315df7c45cb6a0bd0