In commit fb3370b6 (MSVC: Add abstraction for runtime library
selection, 2019-04-10, v3.15.0-rc1~229^2) we overlooked updating flags
for Flang on Windows.  Add them now and update the MSVCRuntimeLibrary
Fortran test to work with Flang.  Base the flags on those we already
use for the GNU-like Clang targeting the MSVC ABI.

Fixes: #19583