Auto merge of #38847 - michaelwoerister:gate-on-incr-comp, r=alexcrichton

travis: Gate on some minimal support for incremental compilation.

This commit adds a travis job that

1. builds a stage2 compiler in incremental mode (but with empty incremental compilation cache), and
2. builds and runs the run-pass test suite also in incremental mode.

Building incrementally with an empty cache makes sure that the compiler doesn't crash in dependency tracking during bootstrapping. Executing the incrementally built test suite gives some measure of confidence that we generate valid code.

Note, however, that the above does not give strong guarantees about the validity of incremental compilation, it just provides a basis for being able to rely on from-scratch incr. comp. builds as reference values in further tests (which then do actual incremental compilation).

r? @alexcrichton

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