Add a test that we don't store stale modtime in modules

The change from r209195 turned out to be important to avoid saving stale
modification time/expected size information in a module file when there
are 3 or more modules in a dependency chain and the bottom one is
rebuilt. So add a test for that.

rdar://problem/17038180

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@209682 91177308-0d34-0410-b5e6-96231b3b80d8