swift234

[frontend] If there is a primary input, only call performTypeChecking() on the primary input.

This has the potential to significantly reduce compile times in primary file
mode: for example, compiling Song.swift from the Name That Tune example as the
primary file previously took ~2.0s with a release build on my machine; with this
change, it now only takes ~0.8s. (The old compile time was presumably dominated
the time it takes to type-check AppDelegate.swift, which is a larger file.)

Swift SVN r13140