[Runtime] Simplify ProtocolConformanceDescriptor::getWitnessTable().

Use SubstGenericParametersFromMetadata to handle substitutions when
checking generic requirements, extending SubstGenericParametersFromMetadata
for this purpose.

Author: DougGregor

stdlib/public/runtime/MetadataLookup.cpp

@@ -1223,6 +1223,12 @@ swift_getTypeByMangledNameImpl(const char *typeNameStart, size_t typeNameLength,
   return swift_checkMetadataState(MetadataState::Complete, metadata).Value;
 }
 
+const Metadata *
+SubstGenericParametersFromMetadata::operator()(unsigned flatIndex) const {
+  // FIXME: Adjust for non-key arguments.
+  return base->getGenericArgs()[flatIndex];
+}
+
 const Metadata *
 SubstGenericParametersFromMetadata::operator()(
                                         unsigned depth, unsigned index) const {
@@ -1259,6 +1265,7 @@ SubstGenericParametersFromMetadata::operator()(
   if (index >= currentContext->getNumGenericParams())
     return nullptr;
 
+  // FIXME: Adjust for non-key arguments.
   return base->getGenericArgs()[flatIndex];
 }
 

stdlib/public/runtime/Private.h

@@ -248,6 +248,7 @@ class TypeInfo {
     explicit SubstGenericParametersFromMetadata(const Metadata *base)
       : base(base) { }
 
+    const Metadata *operator()(unsigned flatIndex) const;
     const Metadata *operator()(unsigned depth, unsigned index) const;
   };
 

stdlib/public/runtime/ProtocolConformance.cpp

@@ -177,26 +177,11 @@ ProtocolConformanceDescriptor::getWitnessTable(const Metadata *type) const {
 
   case ConformanceFlags::ConformanceKind::ConditionalWitnessTableAccessor: {
     // Check the conditional requirements.
-    std::vector<unsigned> genericParamCounts;
-    (void)_gatherGenericParameterCounts(type->getTypeContextDescriptor(),
-                                        genericParamCounts);
-    auto genericArgs = type->getGenericArgs();
+    SubstGenericParametersFromMetadata substitutions(type);
     std::vector<const void *> conditionalArgs;
     bool failed =
       _checkGenericRequirements(getConditionalRequirements(), conditionalArgs,
-        [&](unsigned flatIndex) {
-          // FIXME: Adjust for non-key type parameters.
-          return genericArgs[flatIndex];
-        },
-        [&](unsigned depth, unsigned index) -> const Metadata *  {
-          // FIXME: Adjust for non-key type parameters.
-          if (auto flatIndex = _depthIndexToFlatIndex(depth, index,
-                                                      genericParamCounts)) {
-            return genericArgs[*flatIndex];
-          }
-
-          return nullptr;
-        });
+                                substitutions, substitutions);
     if (failed) return nullptr;
 
     return getWitnessTableAccessor()(