Use the `%target-swift-5.1-abi-triple` substitution to compile the tests for
deployment to the minimum OS versions required for use of _Concurrency APIs,
instead of disabling availability checking.
The instructions related to producing DynamicSelf metadata are always placed at
the start of the function. This creates an issue: which debug location should be
used in order to preserve a sane line table? The current implementation just
uses w/e debug location is present in the new insertion point, which could, for
example, be a debug_declare whose location is not suitable for these purposes.
This patch addresses the issue by removing the debug location of instructions
associated with producing DynamicSelfMetadata.
rdar://120408665
