Make `SourceMap` available for early debug-printing of `Span`s

Normally, we debug-print `Spans` using the `SourceMap` retrieved from
the global `TyCtxt`. However, we fall back to printing out the `Span`'s
raw fields (instead of a file and line number) when we try to print a
`Span` before a `TyCtxt` is available. This makes debugging early phases
of the compile, such as parsing, much more difficult.

This commit stores a `SourceMap` in `rustc_span::GlOBALS` as a fallback.
When a `TyCtxt` is not available, we try to retrieve one from `GLOBALS`
- only if this is not available do we fall back to the raw field output.

I'm not sure how to write a test for this - however, this can be
verified locally by setting `RUSTC_LOG="rustc_parse=debug"`, and
verifying that the output contains filenames and line numbers.

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