Implement the Microsoft __if_exists/if_not_exists extension in initializer-list.

Necessary to parse Microsoft ATL code.

Example: 
  int array[] = {
    0, 
    __if_exists(CLASS::Type) {2, }
    3
  };

will declare an array of 2 or 3 elements depending on if CLASS::Type exists or not.

llvm-svn: 146447