Demangling: always build with NDEBUG

[compnerd]

Avoid a dependency on LLVMSupport at runtime through the llvm_unreachable.
This would pull in llvm_unreachable_internal in debug builds, which requires a
runtime dependency on LLVMSupport which increases the size of the binary
considerably. Always build with NDEBUG. Note that this effectively always
builds the Demangling library WITHOUT asserts :-(.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

CC: @jrose-apple @eeckstein

[compnerd]

@swift-ci please test

[jrose-apple]

I don't like this. Can we use something other than llvm_unreachable instead?

[compnerd]

@jrose-apple - we could use LLVM_BUILTIN_UNREACHABLE to indicate to the compiler that the switch is exhaustive, but we would lose the message (which we do anyways with this, so its not so terrible).

[slavapestov]

I would much rather keep the asserts and fix this problem some other way.

[compnerd]

@slavapestov - I really don't like the idea of providing an alternative implementation of llvm_unreachable_internal.

[compnerd]

At least the build will be aborted with this version. @slavapestov - suggestions welcome :-)

[jrose-apple]

Okay, actually all of these unreachables come after default: return false, so it shouldn't matter. We wouldn't even need them if we always compiled with Clang. This seems fine to me (but Erik should still take a look).

[slavapestov]

We could use assert(false && ...) instead of llvm_unreachable in the demangler.

[compnerd]

@slavapestov - yeah, I suppose assert should be a strong enough hint. I'm gonna let @eeckstein make the call between leaving the LLVM_BUILTIN_UNREACHABLE and assert(0 && "...");.

[eeckstein]

Using assert sounds good to me

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - f133a0120f489425d3e92921afb34cdfda4df6aa

[compnerd]

@swift-ci please test and merge

[compnerd]

@swift-ci please test and merge

[compnerd]

@swift-ci please test macOS platform