The way that we include COMPATIBILITY_OVERRIDE_INCLUDE_PATH freaks out the
syntax highlighting of editors like emacs. It causes the whole file to be
highlighted like it is part of the include string.
To work around this, this patch creates a separate file called
CompatibilityOverrideIncludePath.h that just includes
COMPATIBILITY_OVERRIDE_INCLUDE_PATH. So its syntax highlighting is borked, but
at least in the actual files that contain real code, the syntax highlighting is
restored.
The async main drain queue function is noreturn, but was emitting a
warning due to the override compatibility returning the result of the
overridden function in the wrapper override function. To work around
this, I've added the `OVERRIDE_TASK_NORETURN` macro, which provides an
override point for noreturn functions in the concurrency library that
doesn't return the result from the wrapped function, avoiding the
warning. In the event that the function is not set, the macro is set to
the normal `OVERRIDE` with the return type set to `void`.
Take the existing CompatibilityOverride mechanism and generalize it so it can be used in both the runtime and Concurrency libraries. The mechanism is preprocessor-heavy, so this requires some tricks. Use the SWIFT_TARGET_LIBRARY_NAME define to distinguish the libraries, and use a different .def file and mach-o section name accordingly.
We want the global/main executor functions to be a little more flexible. Instead of using the override mechanism, we expose function pointers that can be set by the compatibility library, or by any other code that wants to use a custom implementation.
rdar://73726764
