[Demangler] Remove and replace DEMANGLER_ALWAYS_ASSERT

[kastiglione]

Originally added in #41452, DEMANGLER_ALWAYS_ASSERT is resulting in crashes in LLDB. This reverts to a regular assert.

rdar://97035347

[kastiglione]

@swift-ci smoke test

[al45tair]

I don't think this is the right thing to do. The reason I added the always-assert in the first place is that we were seeing occasional instances of this in the runtime, and we actively want that to crash because it's a bug and we need to track it down.

If it's causing problems for LLDB, where I think I'd agree that we don't want to crash, maybe it should be ALWAYS_ASSERT_BUT_NOT_IN_LLDB_SOMEHOW.

[al45tair]

To be clear, we want this to crash in the runtime, and we want it to crash in the compiler. I agree that crashing in LLDB is not desirable.

[kastiglione]

From #41452:

We need this in order to gather relevant information when this situation arises;

Is the information gathering still active? This presentation made me think this assert was temporary and could perhaps be removed.

If it's causing problems for LLDB, where I think I'd agree that we don't want to crash, maybe it should be ALWAYS_ASSERT_BUT_NOT_IN_LLDB_SOMEHOW.

I'll look into this.

[al45tair]

Fundamentally, if we're hitting this assertion, it means there's a bug, so if you're still seeing it occasionally then we haven't found all of the causes of it yet. It shouldn't be happening.