[Demangler] Fix assertion failure.

[al45tair]

It's illegal to call node->addChild() with a NULL child argument; it's possible to construct unexpected Node trees by passing invalid manglings, and in this case that was causing popTypeAndGetChild() to fail (because the top node was not a Type node), which then meant that the call to addChild had a NULL child argument.

The simplest fix is to use createWithChildren() to do the node construction, because that function checks its arguments for NULLs.

rdar://125350219

[al45tair]

This is a cherry pick of #72653.

[al45tair]

@swift-ci Please test

[al45tair]

Explanation: The new demangleLifetimeDependenceKind code called addChild() with the result of popTypeAndGetChild(), but the latter can return NULL if the node tree has been constructed from a bad mangled name, and in that case we'd call addChild() with a NULL pointer. (Historically that was allowed, but I changed it to assert always, even in non-asserts builds, because landing an unexpected NULL in the node tree typically causes a crash much later on in a context where we can't easily find out what added it.)
Original PR: #72653
Reviewed by: @meg-gupta
Resolves: rdar://125350219
Tests: I've added a test that fails before this change and passes after it.