Fix issue-268: change return type from non-const to const

[lozinska]

Description

Update return type of locale& to const locale&

Fixes #268.

Checklist

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• Identifiers in product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 or there are no product code changes.
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• These changes introduce no known ABI breaks (adding members, renaming
  members, adding virtual functions, changing whether a type is an aggregate
  or trivially copyable, etc.).
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  the C++ Working Draft (including any cited standards), other WG21 papers
  (excluding reference implementations outside of proposed standard wording),
  and LWG issues as reference material. If they were derived from a project
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[StephanTLavavej]

Thanks! This changes our implementation to do what the Standard says, and I've verified with Wandbox and Compiler Explorer that libstdc++ and libc++ behave identically. There's a very low risk of source breakage from this change - assignment expressions are sometimes used in chained assignments (where this would continue to work) and occasionally used in other expressions, but typically not in modifying ways. I believe we should go ahead and merge this; we can always go back to the old signature if and when an LWG issue is filed and resolved.

[BillyONeal]

Thanks for your contribution!

[StephanTLavavej]

I'm porting this to our Microsoft-internal repo in order to run a full build and test pass.

[StephanTLavavej]

Everything passed, thanks again for the fix!