Regardless of any flags, the stdlib will have its generic metadata
prespecialized.
Temporarily reintroduced the flag to enable the feature flag while
preserving the flag to disable it and changed the default back to off
for the moment.
Temporarily disable metadata prespecialization on platforms other than
MacOS, iOS, tvOS, watchOS, or Linux. At the moment, tests are failing
on Windows with linker errors such as
demangleToMetadata-558ea9.o : error LNK2001: unresolved external symbol $ss5Int64VN
demangleToMetadata-558ea9.o : error LNK2001: unresolved external symbol $sSSSHsWP
Once the issue leading to those linker errors has been resolved, the
feature will be enabled on Windows.
Previously, -Xfrontend -prespecialize-generic-metadata had to be passed
in order for generic metadata to be prespecialized. Now it is
prespecialized unless -Xfrontend
-disable-generic-metadata-prespecialization is passed.
