Fix duplicate directory separator in --remap-path-prefix.
The compiler will currently emit duplicate directory separators when `--remap-path-prefix` has an exact match of the working directory and it is invoked with a relative path to the main source file. For example
```bash
rustc src/main.rs -Cdebuginfo=2 --remap-path-prefix="$(pwd)=abc"
```
will produce the path `abc//src/main.rs` in debuginfo. This is because `some_path.join("")` will append a directory separator to `some_path` and then LLVM does not check if the working directory already ends a directory separator before concatenating it with the relative path.
Dylan DPC
548fca6927