[patch] Simplify non-inline function definitions for std::unordered_xxx containers

[Jonathan Wakely]

While working on PR57272 for unordered containers I was getting a
headache reading all the return types with nested-name-qualifiers
split over three or four lines.

This patch replaces them with trailing return types, so the names used
in the return type are in scope and don't need to be qualified. (This
shaves over 2kB from the file as well as reducing the visual noise.)

There are also some whitespace changes not shown in this patch.

Tested x86_64-linux + powerpc64-linux, committed to trunk.
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