Ratchet up travis to build stage1 and our own LLVM.

Tidy is still run first for failing fast on the easy stuff.

To accomplish this we have travis actually persist ccache across builds. This
has LLVM built within 6 minutes, and all of stage1 built within 18.
Caching should work on fresh PRs (cache acquired from the master branch).
Because all we persist is ccache, there is minimal danger of persisting corrupt
build state.

I had to mangle `configure` a bit to make --enable-ccache work when custom
compilers are provide via CC and CXX.