Sema: Move conformance checking PrettyStackTrace to a better place

Not all conformance checking goes through checkConformance() so we
were losing useful information in some backtraces.

Author: slavapestov

lib/Sema/TypeCheckProtocol.cpp

@@ -1623,6 +1623,8 @@ static void diagnoseConformanceImpliedByConditionalConformance(
 ProtocolConformance *MultiConformanceChecker::
 checkIndividualConformance(NormalProtocolConformance *conformance,
                            bool issueFixit) {
+  PrettyStackTraceConformance trace(TC.Context, "type-checking", conformance);
+
   std::vector<ValueDecl*> revivedMissingWitnesses;
   switch (conformance->getState()) {
     case ProtocolConformanceState::Incomplete:
@@ -4392,8 +4394,6 @@ void TypeChecker::useBridgedNSErrorConformances(DeclContext *dc, Type type) {
 }
 
 void TypeChecker::checkConformance(NormalProtocolConformance *conformance) {
-  PrettyStackTraceConformance trace(Context, "type-checking", conformance);
-
   MultiConformanceChecker checker(*this);
   checker.addConformance(conformance);
   checker.checkAllConformances();