Refactor zombie reporting to use OpSource and prepare for SPIR-T diagnostics.

[eddyb]

This PR has roughly three broad stages:

1. standard SPIR-V OpSource is used instead of rustc SourceFile hashes
   • suboptimal because it keeps Rust source code in the SPIR-V files
     • impact greatly redunced thanks to spirv-builder: force a single CGU (codegen-unit). #1035
     • the OpSources still get stripped out out at the very end by default
     • there's sadly no standard way (even with NonSemantic debuginfo extensions) to use the paths+hashes approach in SPIR-V
   • rustc SourceFiles can still be accurately regenerated from the paths+sources
   • using the standard OpSource mechanism might allow other tools to work well long-term
2. better reporting of the "zombie" (deferred errors) usage stack trace
   • (see also below for a longer example)
   • mostly using information that was already there (just cumbersome to extract outside of SPIR-T)
   • this PR also replaces all "error in use code, zombie in core & friends" logic to always zombie, as the errors are on average higher-quality now for zombies
3. a SPIR-T pass for reporting diagnostics, along the above lines
   • only zombies for now (gated behind --no-early-report-zombies, which makes it replace the existing zombie reporting pass, i.e. the "early zombie reporting")
   • this is most of what's needed to integrate Allow embedding diagnostics into SPIR-T as attributes. spirt#22

---

For 2. we have this change in diagnostics:

before this PR

after this PR

error: Cannot memcpy dynamically sized data --> $CORE_SRC/intrinsics.rs:2460:9 | 2460 | copy(src, dst, count) | ^^^^^^^^^^^^^^^^^^^^^ | = note: Stack: core::intrinsics::copy:: ptr_copy::copy_via_raw_ptr ptr_copy::main main

error: cannot memcpy dynamically sized data --> $CORE_SRC/intrinsics.rs:2460:9 | 2460 | copy(src, dst, count) | ^ | note: used from within `core::intrinsics::copy::` --> $CORE_SRC/intrinsics.rs:2447:21 | 2447 | pub const unsafe fn copy(src: *const T, dst: *mut T, count: usize) { | ^ note: called by `ptr_copy::copy_via_raw_ptr` --> $DIR/ptr_copy.rs:30:18 | 30 | unsafe { core::ptr::copy(src, dst, 1) } | ^ note: called by `ptr_copy::main` --> $DIR/ptr_copy.rs:35:5 | 35 | copy_via_raw_ptr(&i, o); | ^ note: called by `main` --> $DIR/ptr_copy.rs:33:1 | 33 | #[spirv(fragment)] | ^