RFC: Enable Lit testing on Windows buildbot

Hello,

Shall we enable "make check" on msys-mingw hosts?
(I suggest llvm-gcc-native-mingw32-win7 could do)

I expect r129122 must pass all tests on msys-mingw.
(it is llvm+clang's "make check-all")

Testing Time: 146.64s
  Expected Passes : 8297
  Expected Failures : 69
  Unsupported Tests : 557

Thank you, ...Takumi

ps. I am using a script to kill zombie processes on msys;

#!/usr/bin/perl
open($h, "/bin/ps |") || die;
while (<$h>) {
    next unless m=[^0-9]+(\d+).+/mingw/bin/make=;
    system("kill -KILL $1");
}