defined or defined identically before there will not be any
change in functionality.

MinGW-w64 defines __GNUC_VA_LIST as

  #define __GNUC_VA_LIST
  
which is different than the definition here, causing
a warning without the guard.
 


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212183 91177308-0d34-0410-b5e6-96231b3b80d8