swift234

[Driver] Check the mtimes of external dependencies when building.

After we've added all files that are explicitly out of date, check the set
of external dependencies in the graph and see if any of them have been
modified more recently than the oldest object file (or, if an object file
is missing, the corresponding source file; see previous commit). If so,
mark that external dependency as dirty and schedule anything touched by that.

In practice, due to the way external dependencies are collected, this will
almost always lead to a full rebuild. However, the way this is structured
is semantically correct even if that were not the case: an external
dependency is a cascading dependency like any other.

One particular point of information: normal cascading dependencies can be
discovered retroactively, i.e. after a particular source file has already
been compiled. Can that happen for external dependencies? In theory, yes,
due to the leakiness of imports within a module. (If a.swift loads a module
with an extension on String, that extension will be visible to b.swift in
the same module, even though it shouldn't be.) But that's true even if the
external dependency /hasn't/ changed. Given that it's something we consider
a flaw (if low-priority: rdar://problem/16154294), and that it would be
harmless in most actual circumstances, I don't think we should actually
force a full rebuild if one file's imports change.

This completes rdar://problem/19270920

Swift SVN r24337