[llvm-rc] Add optional serialization support for DIALOG(EX) resources.

This is part 5 of llvm-rc serialization support.

This allows DIALOG and DIALOGEX to serialize if dialog-specific optional
statements are provided. These are (as of now): CAPTION, FONT, and
STYLE.

Notably, FONT statement can take more than two arguments when describing
DIALOGEX resources (as in
msdn.microsoft.com/en-us/library/windows/desktop/aa381013.aspx). I made
some changes to the parser to reflect this fact.

Patch by Marek Sokolowski
Differential Revision: https://reviews.llvm.org/D37864

llvm-svn: 315104