swift234

[Driver] Add support for interface-hash.

Dependents of modified files are no longer rebuilt by default, only if it turns
out that file's interface has changed. There is a flag
-driver-always-rebuild-dependents to override this, but we expect it to only be
used for testing. (Most of the existing dependency tests pick up this option;
the two new tests have "interface-hash" in the name.)

This is the second half of ChrisW's work on interface hashing.

rdar://problem/15352929

Swift SVN r30478