Fix IRGen to pass complete metadata to various concurrency builtins

[rjmccall]

Type metadata can be initialized to varying levels of completeness. Most uses of type metadata in the language require "complete" metadata, which ensures that the type's layout is fully initialized. At the opposite end of the spectrum is "abstract" metadata, which basically just allocates the metadata and then fills in the identity of the type. Using abstract metadata when complete metadata is required can easily lead to memory corruption and crashes. Nonetheless, incomplete metadata is important because it allows self-referential types to be built, like a struct S that contains a (resilient) Producer<S> stored property, or a class C whose superclass is Super<C>.

The code for the builtins behind async let, task groups, and Task.runInline was incorrectly requesting abstract type metadata for the result type of the task, which could lead to crashes in the runtime if the type metadata happened to not already have been completed. Task.runInline is a non-standard operation that doesn't appear to be inlinable, and I couldn't convince the compiler to directly emit the task group builtin in user code; this means that those two bugs have probably skated by because the use in the runtime is always passing a value it received as a generic argument, and type metadata passed as generic arguments are required to already be complete. The async let case is definitely possible in user code, unfortunately.

Because the metadata runtime is generally rather eager about completing type metadata, and because it doesn't matter what level of initialization you request if the metadata is already complete, it can be rather hard to encounter this bug; the most likely scenario is a race between threads to request the metadata for the first time.

Fixes rdar://146155888

[rjmccall]

@swift-ci Please test