[Driver, Incremental] Add enable- and disable- only-one-dependency-file flags on-by-default

[davidungar]

Only enabled if the driver is passed -enable-only-one-dependency-file

Since every make-style dependency file has the same contents, it wastes time to output one for each source file. Only write one real one, and create empty dummy files (to keep the build system happy) for the rest.

Latest version of this facility. Intervenes earlier in the driver in order to work well with batch mode.

[davidungar]

@swift-ci please test

[davidungar]

@swift-ci please test source compatibility

[davidungar]

Explanation: Remedies an incremental performance issue, can be disabled with a flag.
Scope of issue: Significant for incremental Swift compilations that involve hundreds of files
Origination: At some point compiler started including system headers in dependency files.
Risk: Rests on the assumption that all make-style dependency files contain the same info and that the build system reads them all.
Reviewed by: Robert Widman
Testing: Normal regression tests.
Radar: rdar://57824038
Note: This may cause problems on Windows: @drodriguez has reported that #28743 caused Windows CI to start failing https://ci-external.swift.org/job/oss-swift-windows-x86_64/2293/ . I'm not sure how to proceed in that regard.

[davidungar]

@swift-ci please test

[davidungar]

@swift-ci please test source compatibility

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 199fa51

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 199fa51

[davidungar]

@swift-ci please nominate