Merging r354633:

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r354633 | rsmith | 2019-02-21 15:04:35 -0800 (Thu, 21 Feb 2019) | 3 lines

Use _Q as MS ABI mangling for char8_t.

Thanks to Yuriy Solodkyy for letting us know the mangling here.
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llvm-svn: 364487