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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@146447 91177308-0d34-0410-b5e6-96231b3b80d8