[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

llvm/utils/gn/build/BUILD.gn

@@ -283,7 +283,7 @@ config("compiler_defaults") {
       "-Werror=date-time",
     ]
     if (current_os == "win") {
-      cflags += [ "-fmsc-version=1916" ]
+      cflags += [ "-fmsc-version=1920" ]
       if (use_lld) {
         cflags += [ "/Brepro" ]
         ldflags += [ "/Brepro" ]