[Serialization] Add pretty stacktraces to conformance serialization

This code path can report compiler inconsistencies, so let's make the
pretty stacktrace more useful when debugging. We probably want to make
the assert on witness.getDecl() a hard error too, but this would break
existing projects at this time.

Author: xymus

lib/Serialization/Serialization.cpp

@@ -1440,6 +1440,8 @@ void Serializer::writeASTBlockEntity(
     const NormalProtocolConformance *conformance) {
   using namespace decls_block;
 
+  PrettyStackTraceConformance trace("serializing", conformance);
+
   // The conformance must be complete, or we can't serialize it.
   assert(conformance->isComplete() || allowCompilerErrors());
   assert(NormalConformancesToSerialize.hasRef(conformance));
@@ -1460,6 +1462,9 @@ void Serializer::writeASTBlockEntity(
   });
 
   conformance->forEachValueWitness([&](ValueDecl *req, Witness witness) {
+      PrettyStackTraceDecl traceValueWitness(
+          "serializing value witness for requirement", req);
+
       ++numValueWitnesses;
       data.push_back(addDeclRef(req));
       data.push_back(addDeclRef(witness.getDecl()));