[Explicit Module Builds][Incremental Builds] Only re-build module dependencies which have changed or whose dependencies have changed.

[artemcm]

Up until now, incremental explicit builds behaved on an all-or-nothing basis: if any of the dependencies have changed, or new dependencies were added, then the driver launches a full dependency scan, followed by re-building all dependency modules.

Now, instead, we take a fine-grained approach to rebuilding module dependencies. The overall incremental build flow w.r.t. explicit module dependencies is now as-follows:

1. If there is a prior inter-module dependency graph and the source import set has not changed, and all dependencies are up-to-date, skip the dependency scan and re-use the prior graph.
2. When computing the first compilation wave's mandatory pre-compile jobs, examine each module dependency in the graph (either re-used prior or newly-computed). If any of the module's inputs are newer than the output, or if the output does not exist, this module is invalidated and must be re-compiled. Necessarily, every module in the graph between the root and the invalidated module must also be invalidated and re-compiled.

(Note: that we do not need to concern ourselves with changes to the compile command of a given module changing from the prior build - if that happened, then the module output path will have changed to reflect the change in the context hash, and expected output will be missing at the new expected path.)

Resolves rdar://113638007

[artemcm]

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