This has been an unnecessary code path for a long time now and should be removed particularly because it triggers wasteful `stat` calls.
rdar://51523161
- Allow them to use substitutions.
- Consistently use 'a' as a mangling operator.
- For generic typealiases, include the alias as context for any generic
parameters.
Typealiases don't show up in symbol names, which always refer to
canonical types, but they are mangled for debug info and for USRs
(unique identifiers used by SourceKit), so it's good to get this
right.
The root cause is that NormalProtocolConformance::forEachValueWitness()
needs to skip protocol members that are not requirements.
Otherwise we end up passing such a non-requirement member down to
NormalProtocolConformance::getWitness() and hit an assert when we
cannot find it.
It looks like this code path was only ever hit from SourceKit.
The fix moves TypeChecker::isRequirement() to a method on ValueDecl,
and calls it in the right places.
Fixes <https://bugs.swift.org/browse/SR-3815>.
