Initial build mode build emits twice

[amcasey]

From #42173 (comment)

In the course of investigating #42173, I noticed that a clean build of the largest project spent 50 seconds on emit. I picked out the slowest files and noticed that each occurred twice. It turns out that updateShapeSignature triggers a separate emit from the real one (fortunately, with no file writing), which accounts for 22 seconds of the emit time. Note that this was with TS 4.2 - 4.3 skips signature computation on a clean build.

It seems like we could potentially realize a very significant perf win if we could combine the two emit passes. I believe @sheetalkamat has already been thinking about ways to do this.