Revert "Disallow ArrayRef assignment from temporaries."

This reverts commit r283798, as it causes static asserts on
MSVC 2015 with the following errors:

ArrayRefTest.cpp(38): error C2338: Assigning from single prvalue element
ArrayRefTest.cpp(41): error C2338: Assigning from single xvalue element
ArrayRefTest.cpp(47): error C2338: Assigning from an initializer list

llvm-svn: 283803
2 files changed