rustllvm: Use report_fatal_error instead of llvm_unreachable

[hanna-kruppe]

Some functions in rustllvm rely on llvm_unreachable to catch errors. However, in release mode llvm_unreachable expands to the C++ compiler's equivalent of intrinsics::unreachable() (i.e., it's a hint to the optimizer and UB to execute). This subverts the intent of several functions, causing us to execute UB instead of crashing as intended. All uses of llvm_unreachable in rustllvm should be audited and replaced with report_fatal_error unless the potential-UB is intended.

[Mark-Simulacrum]

Nominating for prioritization. I feel like this might be somewhat severe, and as such should get P-med/P-high but I don't really know what the effects of this can be.

[hanna-kruppe]

In practice, it's been fine so far, so it'll probably be fine for a while longer. I don't think P-high is warranted, honestly. Most uses of llvm_unreachable that I recall are in code that specifically translates between C++ enums and Rust enums to ensure we don't get UB from C++ adding extending their enums without us noticing. That code will actually be fine as long as the corresponding C++ enums don't change.

Besides, instead of "release mode" I should have said "without assertions" (the macro definition depends on NDEBUG). That is, Release+Assert builds (as nightlies currently are, IIRC?) will also crash instead of causing UB.

Still, it's an easy fix, so someone with a little bit of time (coughnot mecough) should do it.

[nikomatsakis]

triage: P-medium

Would be good to do!