print: name and reuse inlined call site debuginfo "scopes".

[eddyb]

This addresses the visual (and functional) bloat caused by accurate (wrt inlining) debuginfo from Rust-GPU,
which on the motivating example, results in these changes:

_{(middle column obtained by making DbgScope::maybe_print_name_as_ref_or_def always return None, which should not be considered likely output after this PR, it's only kept around as a fallback, e.g. if intra-function debuginfo ends up used in Const attributes etc.)}

	Before this PR	New style w/o reuse	After this PR
.spirt line count	190498	190498	46365
.spirt.html file size	96 MiB	95.2 MiB	33.5 MiB
Worst-case comparison

(EDIT: I've sadly had to force widths via explicit <img> tags because GitHub started applying overflow-wrap: anywhere to the entire markdown render output, which means individual letters in the text cells are competing against whole <img>s, during table layout, and the letters end up inevitably all in one vertical line)

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Initially I was hoping for some kind of "stateful" comment system, and/or showing "debug scopes" using braces within the debuginfo comments, but the // d123 = ... notation added in this PR was an algorithmic nightmare as-is. _{(mainly because of multi-version support, where uses of debuginfo can easily disappear/move as nodes get simplified/wholly removed/etc. in various passes, making it hard so share common placements for such debuginfo "definitions")}

Also, it's been aesthetically tricky to handle the transition from "location of (inlined) call site" to "location inside (inlined) callee", and this PR ended up flipping the order, from inside-out to outside-in. _{(you might be able to tell between the first and second column, it used to look like a backtrace that keeps "zooming out", from leaf function to entry-point, whereas now it "zooms in", keeping the leaf-most location closest to whatever follows the debuginfo comments)}

That 33.5 MiB .spirt.html after this PR still makes browsers a bit unhappy. _{(I was going to share more comparisons but even that single one took minutes per screenshot just to scroll with the Firefox screenshot tool!)
(specific HTML size/complexity issues might be fixable, but it's hard to tell what browsers are struggling with most, without actually testing a tentative change)}

However, it's a far cry from what it used to be, and any visual and UX improvements are welcome.
_{(let alone the 216 MiB file I started from, before fixing the format_args! "decompiler" on my Rust-GPU branch, as the output was full of core::fmt internals, esp. kept alive by function pointers/vtables)}

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Not sure how to handle reviews, but I would gladly walk someone through all of this mess if anyone cares.
(Thought, just like the other recent pretty-printer PRs, this is purely about aesthetics/ergonomics, so anything being "wrong" or "imprecise" shouldn't affect the correctness of actual SPIR-T passes etc.)

[Firestar99]

I feel like as this is just debug print, and it massively helps you, we should just merge it without me explicitly reviewing every single part of it