[ADT] Make SmallVector emplace_back return a reference
[ADT] Make SmallVector emplace_back return a reference

This follows the C++17 std::vector change and can simplify immediate
back() calls.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@356312 91177308-0d34-0410-b5e6-96231b3b80d8

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