Re-enable LLVM and debug assertions for slow builders

[alexcrichton]

Assertions are currently disabled on a number of builders:

• test-various - debug asserts, intentional - needed to properly test code size of some tests
• dist-x86_64-apple - llvm, debug
• dist-x86_64-apple-alt - llvm, debug
• x86_64-apple - llvm, debug
• dist-aarch64-apple - llvm, debug

[alexcrichton]

For posterity, this had a pretty massive impact on CI times for the thre platforms that we disabled debug/llvm assertions for:

[aidanhs]

(currently waiting on a potential upgrade to our appveyor builder machines, maybe happening soonish)