[Concurrency] Fix arm64e ptrauth violations in Embedded Swift Concurrency

[kubamracek]

Under arm64e ptrauth, we need to be more diligent on:

• (1) The "minimal value witness table" that IRGen generates for task_create (ResultTypeInfoTaskOptionRecord) needs its function pointers to be signed. This PR uses the same signing scheme as regular Swift's value witness tables, i.e. address-discriminated pointers with special values per witness index (e.g. SpecialPointerAuthDiscriminators::InitializeWithCopy).

• (2) Accessing HeapObject's metadata, which under Embedded Swift is not actually a ClassMetadata object.

rdar://138814877

[kubamracek]

@swift-ci please test

[al45tair]

CMake Error at /Users/ec2-user/jenkins/workspace/swift-PR-macos/branch-main/llvm-project/libcxx/CMakeLists.txt:70 (message):
  Unsupported hardening mode: 'unchecked'.  Supported values are
  none;fast;extensive;debug.

[kubamracek]

@swift-ci please test

[al45tair]

LGTM