[VS2015 (v140)] Loads of "error C2983: '...': all declarations must have an identical __declspec(code_seg(...))"

Affected: VS14 2015 (as in using Toolset v140). (msvc, msvc-amd64, msbuild, vssolution.)
Unaffected: VS15 2017 (v141), VS16 2019 (v141/v142).

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Example:

C:\reactos-cov\win32ss\user\ntuser\timer.c(595): error C2983: 'InitTimerImpl': all declarations must have an identical __declspec(code_seg(...))

c:\reactos-cov\win32ss\user/ntuser/timer.h(35): note: see declaration of 'InitTimerImpl'

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0.4.15-dev-1243-gba44701: Still builds fine.
0.4.15-dev-1244-g658f742: Does not compile.
0.4.15-dev-1245-g5c7ce44: Regressed. Cc extravert34.

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Potential options:

• Stop bothering, so explicitly drop VS2015 support.
• Don't really care, so simply demote C2983 error to a warning. (If that is possible.)
• Actually fix these errors, so restoring/adding identical declarations.
• ...