Before this change, the ABI checker didn't complain about missing @available
attributes on new APIs if the enclosing scopes of new APIs have
availability information. Instead, we should require explicit @available attributes
for new APIs to prevent them inheriting wrong availability information from a
preexisting scope.
rdar://81719628
Otherwise we set it on all targets/languages in a subdirectory (I forgot if it
propagates up). Regardless, this type of viral stuff is something we want to
move away from since it creates a code that is a "forall" piece of code rather
than a piece of code that only effects a single target.
I also conditionalized the actual definitions being added on the compiled file's
language being C,CXX,OBJC,OBJCXX since as we add Swift sources to the host side
of the compiler, we will not want these flags to propagate to Swift sources.
