The witness table had shared linkage, but we weren't serializing them,
which would cause linking errors if we emitted a reference to such a
witness table from a different module than the one where it was first
defined, as a result of deserializing and optimizing SIL.
This issue was introduced when SIL witness table serialization was
made conditional on the -sil-serialize-witness-tables flag, which is
normally only enabled for the standard library.
When the flag was added, existing tests were updated to pass the
flag, which masked the issue. Remove the flag from existing tests,
ensure that imported witness tables are still [serialized], and add
a new test specifically for the behavior enabled by this flag.
Also, add a third [serializable] state for functions whose bodies we
*can* serialize, but only do so if they're referenced from another
serialized function.
This will be used for bodies synthesized for imported definitions,
such as init(rawValue:), etc, and various thunks, but for now this
change is NFC.
Textual SIL was sometimes ambiguous when SILDeclRefs were used, because the textual representation of SILDeclRefs was the same for functions that have the same name, but different signatures.
Textual SIL was sometimes ambiguous when SILDeclRefs were used, because the textual representation of SILDeclRefs was the same for functions that have the same name, but different signatures.
Also, mark witness thunks for [fragile] witnesses as [fragile].
This allows us to serialize the witness table, as well as any thunks
for witnesses declared @_transparent and @inline(__always).
