Conditional conformances aren't quite ready yet for Swift 4.1, so
introduce the flag `-enable-experimental-conditional-conformances` to
enable conditional conformaces, and an error when one declares a
conditional conformance without specifying the flag.
Add this flag when building the standard library (which will vend
conditional conformances) and to all of the tests that need it.
Fixes rdar://problem/35728337.
When emitting a witness table accessor (e.g., an associated type metadata
or associated type conformance accessor) for a conditional conformance,
bind the conditional requirements so we can refer to them within that
accessor. Witness methods get this behavior already through the SelfWitnessTable parameter, but everything the witness table needs it.
Fixes most of SR-6478.
