Add debugPrintf-based panic reporting, controlled via spirv_builder::ShaderPanicStrategy.

[eddyb]

This is a followup to my earlier PR about panics (and the only thing I want to really get into 0.9):

• Lower aborts (incl. panics) to "return from entry-point", instead of infinite loops. #1070

There are better docs on spirv_builder::ShaderPanicStrategy, but this is the basic usage example:

    SpirvBuilder::new("examples/shaders/sky-shader", "spirv-unknown-vulkan1.1")
        .shader_panic_strategy(spirv_builder::ShaderPanicStrategy::DebugPrintfThenExit {
            print_inputs: true,
            print_backtrace: true,
        })

_{(that is, to enable emitting all the information we can right now - print_inputs will show values of entry-point inputs, to identify the invocation, while print_backtrace will collect the static "inlined call frames" information we track for diagnostics, into the message)}

And this is what it looks like with an artificially induced panic in sky-shader (plus some extra functions that just call the next one in the chain, to show off a non-trivial backtrace):

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If you look at the diff, you may notice most of it is to the inliner: yupp it had more bugs, but not only that, the entire strategy for tracking debuginfo was wrong because it's fundamentally an outside-in inliner, instead of the usual inside-out model (I'm honestly surprised any of the past diagnostics demos worked at all).

The other changes I've made were to the ash runner, hopefully they're not too disruptive (cc @DJMcNab).
This is how I was testing (after sprinkling sky-shader with extra panics):

VK_LAYER_ENABLES=VK_VALIDATION_FEATURE_ENABLE_DEBUG_PRINTF_EXT cargo run -p example-runner-ash --release -- -d

Haven't been able to break anything with it, so I'm relatively confident the inliner changes aren't semantically meaningful (after all, the whole point is to tweak debuginfo to make it more accurate, not to change anything else).

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EDIT: also check out this other PR, which further improved the output and figured out how to use it with wgpu:

• example-runner-wgpu: add --force-spirv-passthru and rely on it for debugPrintf. #1036

[eddyb]

For anyone seeing this PR later, the output, wgpu compatibility, and docs, were improved in:

• example-runner-wgpu: add --force-spirv-passthru and rely on it for debugPrintf. #1036