Auto merge of rust-lang#3798 - RalfJung:miri-script-remap-path-prefix, r=saethlin

miri-script: use --remap-path-prefix to print errors relative to the right root

Inspired by rust-lang/rust-clippy#13232, this makes it so that when cargo-miri fails to build, `./miri check` will print errors with paths like `cargo-miri/src/setup.rs`. That means we can get rid of the miri-symlink-hacks and instead tell RA to just always invoke the `./miri clippy` script just once, in the root.

This means that we can no longer share a target dir between cargo-miri and miri as the RUSTFLAGS are different to crates that are shared in the dependency tree need to be built twice with two different flags. `miri-script` hence now has to set the MIRI environment variable to tell the `cargo miri setup` invocation where to find Miri.

I also made it so that errors in miri-script itself are properly shown in RA, for which the `./miri` shell wrapper needs to set the right flags.

Author: bors