Frontend: infer default blocklists to use when the explicit paths aren't given by swift-driver

Although swift-driver always passes down these blocklist for the compiler to consume, some frontend
tools, like ABI checker, are invoked by the build system directly. Therefore, we need to teach
the compiler to infer these blocklist files like prebuilt module cache.

Author: nkcsgexi

include/swift/Frontend/Frontend.h

@@ -232,6 +232,9 @@ class CompilerInvocation {
   ///       options have been parsed.
   void setDefaultPrebuiltCacheIfNecessary();
 
+  /// If we haven't explicitly passed -blocklist-paths, set it to the default value.
+  void setDefaultBlocklistsIfNecessary();
+
   /// Computes the runtime resource path relative to the given Swift
   /// executable.
   static void computeRuntimeResourcePathFromExecutablePath(

lib/Frontend/CompilerInvocation.cpp

@@ -159,6 +159,31 @@ void CompilerInvocation::setDefaultPrebuiltCacheIfNecessary() {
     (llvm::Twine(pair.first) + "preferred-interfaces" + pair.second).str();
 }
 
+void CompilerInvocation::setDefaultBlocklistsIfNecessary() {
+  if (!LangOpts.BlocklistConfigFilePaths.empty())
+    return;
+  if (SearchPathOpts.RuntimeResourcePath.empty())
+    return;
+  // XcodeDefault.xctoolchain/usr/lib/swift
+  SmallString<64> blocklistDir{SearchPathOpts.RuntimeResourcePath};
+  // XcodeDefault.xctoolchain/usr/lib
+  llvm::sys::path::remove_filename(blocklistDir);
+  // XcodeDefault.xctoolchain/usr
+  llvm::sys::path::remove_filename(blocklistDir);
+  // XcodeDefault.xctoolchain/usr/local/lib/swift/blocklists
+  llvm::sys::path::append(blocklistDir, "local", "lib", "swift", "blocklists");
+  std::error_code EC;
+  if (llvm::sys::fs::is_directory(blocklistDir)) {
+    for (llvm::sys::fs::directory_iterator F(blocklistDir, EC), FE;
+         F != FE; F.increment(EC)) {
+      StringRef ext = llvm::sys::path::extension(F->path());
+      if (ext == "yml" || ext == "yaml") {
+        LangOpts.BlocklistConfigFilePaths.push_back(F->path());
+      }
+    }
+  }
+}
+
 static void updateRuntimeLibraryPaths(SearchPathOptions &SearchPathOpts,
                                       llvm::Triple &Triple) {
   llvm::SmallString<128> LibPath(SearchPathOpts.RuntimeResourcePath);
@@ -2934,6 +2959,7 @@ bool CompilerInvocation::parseArgs(
 
   updateRuntimeLibraryPaths(SearchPathOpts, LangOpts.Target);
   setDefaultPrebuiltCacheIfNecessary();
+  setDefaultBlocklistsIfNecessary();
 
   // Now that we've parsed everything, setup some inter-option-dependent state.
   setIRGenOutputOptsFromFrontendOptions(IRGenOpts, FrontendOpts);