[Dependency Scanning] Emit a warning when failing to load a binary module of a non-resilient dependency

This failure will most-likely result in the dependency query failure which will fail the scan. It will be helpful if the scanner emitted diagnostic for each such module it rejected to explain the reason why.

Resolves rdar://142906530

lib/Serialization/SerializedModuleLoader.cpp

@@ -304,6 +304,45 @@ SerializedModuleLoaderBase::getModuleName(ASTContext &Ctx, StringRef modulePath,
   return ModuleFile::getModuleName(Ctx, modulePath, Name);
 }
 
+std::optional<std::string> SerializedModuleLoaderBase::invalidModuleReason(serialization::Status status) {
+  using namespace serialization;
+  switch (status) {
+  case Status::FormatTooOld:
+    return "compiled with an older version of the compiler";
+  case Status::FormatTooNew:
+    return "compiled with a newer version of the compiler";
+  case Status::RevisionIncompatible:
+    return "compiled with a different version of the compiler";
+  case Status::ChannelIncompatible:
+    return "compiled for a different distribution channel";
+  case Status::NotInOSSA:
+    return "module was not built with OSSA";
+  case Status::MissingDependency:
+    return "missing dependency";
+  case Status::MissingUnderlyingModule:
+    return "missing underlying module";
+  case Status::CircularDependency:
+    return "circular dependency";
+  case Status::FailedToLoadBridgingHeader:
+    return "failed to load bridging header";
+  case Status::Malformed:
+    return "malformed";
+  case Status::MalformedDocumentation:
+    return "malformed documentation";
+  case Status::NameMismatch:
+    return "name mismatch";
+  case Status::TargetIncompatible:
+    return "compiled for a different target platform";
+  case Status::TargetTooNew:
+    return "target platform newer than current platform";
+  case Status::SDKMismatch:
+    return "SDK does not match";
+  case Status::Valid:
+    return std::nullopt;
+  }
+  llvm_unreachable("bad status");
+}
+
 llvm::ErrorOr<llvm::StringSet<>>
 SerializedModuleLoaderBase::getMatchingPackageOnlyImportsOfModule(
     Twine modulePath, bool isFramework, bool isRequiredOSSAModules,
@@ -504,7 +543,8 @@ SerializedModuleLoaderBase::resolveMacroPlugin(const ExternalMacroPlugin &macro,
 
 llvm::ErrorOr<ModuleDependencyInfo>
 SerializedModuleLoaderBase::scanModuleFile(Twine modulePath, bool isFramework,
-                                           bool isTestableImport) {
+                                           bool isTestableImport,
+                                           bool isCandidateForTextualModule) {
   const std::string moduleDocPath;
   const std::string sourceInfoPath;
 
@@ -521,10 +561,19 @@ SerializedModuleLoaderBase::scanModuleFile(Twine modulePath, bool isFramework,
 
   if (Ctx.SearchPathOpts.ScannerModuleValidation) {
     // If failed to load, just ignore and return do not found.
-    if (loadInfo.status != serialization::Status::Valid) {
+    if (auto loadFailureReason = invalidModuleReason(loadInfo.status)) {
+      // If no textual interface was found, then for this dependency
+      // scanning query this was *the* module discovered, which means
+      // it would be helpful to let the user know why the scanner
+      // was not able to use it because the scan will ultimately fail to
+      // resolve this dependency due to this incompatibility.
+      if (!isCandidateForTextualModule)
+        Ctx.Diags.diagnose(SourceLoc(), diag::dependency_scan_module_incompatible,
+                           modulePath.str(), loadFailureReason.value());
+
       if (Ctx.LangOpts.EnableModuleLoadingRemarks)
-        Ctx.Diags.diagnose(SourceLoc(), diag::skip_module_invalid,
-                           modulePath.str());
+        Ctx.Diags.diagnose(SourceLoc(), diag::dependency_scan_skip_module_invalid,
+                           modulePath.str(), loadFailureReason.value());
       return std::make_error_code(std::errc::no_such_file_or_directory);
     }