[Support] Workaround compiler bug in MSVC

https://developercommunity.visualstudio.com/t/Prev-Issue---with-__assume-isnan-/1597317

This was causing unittest failures on Windows for the GitHub actions
based CI we use in the release branches.

Failed Tests (2):
  LLVM-Unit :: Support/./SupportTests.exe/FormatVariadicTest.BigTest
  LLVM-Unit :: Support/./SupportTests.exe/NativeFormatTest.BoundaryTests

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D129822