Fix ConvertAndFreeCoTaskMemString for 32 bit platforms

[jiria]

This is submitted on behalf of askariv.

Based on align.go line 367:
if Widthptr == 4 && w != int64(int32(w)) {
yyerror("type %v too large”, t)
}
The idea in this case is to convert to a slice of a large array of uint16s so it can be used with practically any size of a string, since ‘func UTF16ToString(s []uint16) string’.
The original code had [1<<30] of uint16 elemets which make the width 1 << 31 or 2147483648 (0x80000000) meaning -2147483648 in 32bit!
This is why [1<<29] works, and it is large enough for any real purpose, after all it is a string.

Please let us know if you have a better idea on how to do the type cast.

[jterry75]

LGTM. @jstarks would know if this is the best fix but I think it works.