[SourceKit] Allow module references to be indexed (#19243)

Module references get indexed as a 'module' symbol; they get USRs similar to how clang would assign a USR for a module reference.

JIRA: https://bugs.swift.org/browse/SR-8677

lib/AST/Module.cpp

@@ -365,6 +365,10 @@ ModuleDecl::ModuleDecl(Identifier name, ASTContext &ctx)
   setAccess(AccessLevel::Public);
 }
 
+bool ModuleDecl::isClangModule() const {
+  return findUnderlyingClangModule() != nullptr;
+}
+
 void ModuleDecl::addFile(FileUnit &newFile) {
   // Require Main and REPL files to be the first file added.
   assert(Files.empty() ||
@@ -1636,6 +1640,13 @@ const ModuleDecl* ModuleEntity::getAsSwiftModule() const {
   return nullptr;
 }
 
+const clang::Module* ModuleEntity::getAsClangModule() const {
+  assert(!Mod.isNull());
+  if (Mod.is<const ModuleDecl*>())
+    return nullptr;
+  return getClangModule(Mod);
+}
+
 // See swift/Basic/Statistic.h for declaration: this enables tracing SourceFiles, is
 // defined here to avoid too much layering violation / circular linkage
 // dependency.