libSyntax: specialize struct declaration syntax node. (#13307)

To construct struct syntax, this patch first specialized type
inheritance clause. For protocol's class requirement, we currently
treat it as an unknown type.

This patch also teaches SyntaxParsingContext to collect syntax nodes
from back in place. This is useful to squash multiple decl modifiers
for declarations like function. This is not used for struct declaration
because only accessibility modifier is allowed.

lib/Syntax/SyntaxParsingContext.cpp

@@ -126,6 +126,21 @@ void SyntaxParsingContext::createNodeInPlace(SyntaxKind Kind) {
   }
 }
 
+void SyntaxParsingContext::collectNodesInPlace(SyntaxKind ColletionKind) {
+  assert(isCollectionKind(ColletionKind));
+  assert(isTopOfContextStack());
+  if (!Enabled)
+    return;
+  auto Count = std::count_if(Parts.rbegin(), Parts.rend(),
+                             [&](const RC<RawSyntax> &Raw) {
+    return SyntaxFactory::canServeAsCollectionMember(ColletionKind,
+                                                     make<Syntax>(Raw));
+  });
+  if (Count) {
+    createNodeInPlace(ColletionKind, Count);
+  }
+}
+
 namespace {
 RC<RawSyntax> bridgeAs(SyntaxContextKind Kind, ArrayRef<RC<RawSyntax>> Parts) {
   if (Parts.size() == 1) {