[gn build] Set -fmsc-version=1920 on Windows

Now that the minimum version version of MSVC required to build LLVM has
been bumped, we see

  ../../llvm/include\llvm/Support/Compiler.h(94,2): error: LLVM requires
  at least VS 2019.
  #error LLVM requires at least VS 2019.

e.g. http://45.33.8.238/win/53703/step_4.txt

1920 corresponds to the earliest version of VS 2019.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D118713