MSVC currently doesn't support 80 bits long double. But ICC does support
it on Windows. Besides, there're also some users asked for this feature.
We can find the discussions from stackoverflow, msdn etc.
Given Clang has already support `-mlong-double-80`, extending it to
support for Windows seems worthwhile.
Reviewed By: rnk, erichkeane
Differential Revision: https://reviews.llvm.org/D115441
Phoebe Wang