Streamline `Compiler`#64016

Merged

bors merged 4 commits intorust-lang:masterfrom

nnethercote:Compiler-fiddling

Sep 25, 2019

Commits on Sep 1, 2019

Don't call `self.parse()` in `Compiler::crate_name()` unless necessary.
nnethercote
committed
Remove `lower_to_hir()` call from `prepare_output()`.
nnethercote
committed

Commits on Sep 11, 2019

Move call site of `dep_graph_future()`.
nnethercote
committed

Commits on Sep 18, 2019

Add a comment to `Compiler::compile()`.
nnethercote
committed