Re-land "[dsymutil] Timestmap verification for __swift_ast"

This patch adds timestamp verification for swiftmodule files. A new flag
is provided to allows us to disable this check in order to allow testing
of this feature.

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315684 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed