[Issue 20652] extern(C++) doesn't seem to mangle the types in core.simd right

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20652

--- Comment #5 from Iain Buclaw <ibuclaw at gdcproject.org> ---
According to the testsuite, it looks like the MSVC mangler is broken too, not
just the Itanium one.

DMD: ?test20652@@YAXAET__m128@@@Z
MSVC: ?test20652@@YAXAEBT__m128@@@Z

The B looks to be for const, this has also been addressed in the PR.

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