[lldb] Avoid mangling empty opaque types

[kastiglione]

Update TypeSystemSwiftTypeRef::RemangleAsType to handle empty opaque types. Without this change, assertions will occur when running unit tests with an asserts-build of lldb.

An example of where this happens is Builtin.RawPointer ($sBpD). For values with that type, the typeref is:

(swift::reflection::OpaqueTypeRef *) typeref = 0x000060000309c000 {
  swift::reflection::TypeRef = {
    Kind = Opaque
  }
}

When demangling the above typeref, the resulting demangle tree is an empty OpaqueType:

(swift::Demangle::NodePointer) node = 0x0000000105240420 Type, 1 child {
  NodeKind = Type
  [0] = 0x0000000105240408 OpaqueType, 0 children {
    NodeKind = OpaqueType
  }
}

In Swift's Remangler::mangleOpaqueType, it's expected that the opaque type have 3 children:

DEMANGLER_ASSERT(node->getNumChildren() >= 3, node);

[kastiglione]

@swift-ci test

[kastiglione]

A possible alternative to this is to update Remangler::mangleOpaqueType to accept opaque nodes with no children, but I'm not sure whether that's a valid change to make, or not.

[kastiglione]

@swift-ci test macOS

[kastiglione]

@swift-ci test macOS

[adrian-prantl]

This patch seems fine for avoiding the crash, but: Is there a bug in the remangler that makes it impossible to round-trip $sBpD? If yes, would it be better to fix that instead?