IR backtraces: expand multiple call frames

We currently abort IR backtrace generation when encountering multiple call sites: https://github.com/JuliaGPU/GPUCompiler.jl/blob/d370e190268f02f5636bb08bd1a1c971228c0db5/src/debug.jl#L50

We should probably just generate multiple stackframes instead, which seems strictly more useful.

Start in src/debug.jl around line 50, where IR backtrace generation currently aborts on multiple call sites. Trace the existing backtrace-generation path and make completion produce multiple stackframes rather than aborting; done means multiple call sites are represented in the generated backtrace.

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