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

[aeubanks]

Summary

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.

[thakis]

We should explicitly pass _some_ version, else this picks up something depending on the local system. We want this to be as machine-independent as possible. (Think eg of goma, it might different versions server side and client side.)

[thakis]

(ps: I think the actual error is due to that bot actually having msvc 2017 installed. I tried to update it on the bot but in the process I broke rsync and I've been pretty busy with other stuff this week. Due to that, that bot hasn't had updated builds in a few days.)

[aeubanks]

use 1920

[thakis]

Thanks!

llvm/utils/gn/build/BUILD.gn

if (current_os == "win") {
     ldflags += [ "/pdbaltpath:%_PDB%" ]
   }
   if (is_clang) {
     cflags += [
       "-no-canonical-prefixes",
       "-Werror=date-time",
     ]
     if (current_os == "win") {
-      cflags += [ "-fmsc-version=1916" ]
       if (use_lld) {
         cflags += [ "/Brepro" ]
         ldflags += [ "/Brepro" ]
       }
     }
     if (use_relative_paths_in_debug_info) {
       cflags += [ "-fdebug-compilation-dir=." ]
     }