Paths in save-analysis files are not remapped by --remap-path-prefix

[jsgf]

--remap-path-prefix is supposed to remap all references to paths in all output (error messages, debug info, etc). It looks like this isn't happening for save-analysis files.

For example, in current nightly: rustc 1.29.0-nightly (12ed235 2018-07-18)

This input

pub struct Foo;

rustc -Zsave-analysis -g --crate-type=rlib --emit=metadata t.rs --error-format json --remap-path-prefix=$(pwd)=.

produces a save-analysis file which still has references to my home dir:

        "external_crates": [
            {
                "file_name": "/Users/jsgf/t.rs",
                "id": {
                    "disambiguator": [
                        287500826263317014,
                        5864436369277545838
                    ],
                    "name": "std"
                },
                "num": 1
            },

Example in full output.

This is undesirable because it makes the output depend on the build directory, which breaks build artifact caching.

[jsgf]

cc @nrc @Xanewok