[SR-1142] [llbuild] Don't immediately run scanned dependencies once determined they need to run

[ddunbar]

Previous ID	SR-1142
Radar	rdar://problem/21793691
Original Reporter	@ddunbar
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	llbuild
Labels	Bug
Assignee	None
Priority	Medium

md5: 0f59127cf21fefd6bb4877a8888207a5

Issue Description:

There is a subtle bug in the current dependency scanning where we will immediately start executing a task that we have scanned and realized needs to run, but that may occur before the point at which the downstream task of which it is a dependent has actually requested it.

This generally only shows up when making us of dynamic dependencies, which is why it hasn't been noticed until now.

As a concrete example, suppose a build executes in which
C executes, computes A, realizes based on A that it needs B
so we end up with
C depends { A, B }

Now suppose that the a new build is triggered, in which the result of A differs:
C executes, computes A, realizes based on A that it needs B'

In this situation, the current dependency scanning may also traverse to B, and trigger it to rebuild even though it is not actually used.