AST: Request-ify ValueDecl::findImport().

To prevent performance regressions from calling `findImport()` more frequently,
request-ify the computation.

NFC.

Prerequisite of rdar://16154294

lib/AST/Module.cpp

@@ -2572,6 +2572,38 @@ SourceFile::setImports(ArrayRef<AttributedImport<ImportedModule>> imports) {
   Imports = getASTContext().AllocateCopy(imports);
 }
 
+std::optional<AttributedImport<ImportedModule>>
+SourceFile::findImport(const ModuleDecl *module) const {
+  return evaluateOrDefault(getASTContext().evaluator,
+                           ImportDeclRequest{this, module}, std::nullopt);
+}
+
+std::optional<AttributedImport<ImportedModule>>
+ImportDeclRequest::evaluate(Evaluator &evaluator, const SourceFile *sf,
+                            const ModuleDecl *module) const {
+  auto &ctx = sf->getASTContext();
+  auto imports = sf->getImports();
+
+  // Look to see if the owning module was directly imported.
+  for (const auto &import : imports) {
+    if (import.module.importedModule == module)
+      return import;
+  }
+
+  // Now look for transitive imports.
+  auto &importCache = ctx.getImportCache();
+  for (const auto &import : imports) {
+    auto &importSet = importCache.getImportSet(import.module.importedModule);
+    for (const auto &transitive : importSet.getTransitiveImports()) {
+      if (transitive.importedModule == module) {
+        return import;
+      }
+    }
+  }
+
+  return std::nullopt;
+}
+
 bool SourceFile::hasImportUsedPreconcurrency(
     AttributedImport<ImportedModule> import) const {
   return PreconcurrencyImportsUsed.count(import) != 0;