[dsymutil] Timestmap verification for __swift_ast

This patch adds timestamp verification for swiftmodule files.

 - A new flag is provided to allows us to continue testing of the code
   for embedding the__swift_ast. (git doesn't maintain timestamps)
 - Adds a new test for fat (arm) binaries.

Differential revision: https://reviews.llvm.org/D38686

llvm-svn: 315456