Index: lldb/trunk/include/lldb/Host/macosx/HostInfoMacOSX.h
===================================================================
--- lldb/trunk/include/lldb/Host/macosx/HostInfoMacOSX.h
+++ lldb/trunk/include/lldb/Host/macosx/HostInfoMacOSX.h
@@ -38,6 +38,8 @@
   static bool ComputeHeaderDirectory(FileSpec &file_spec);
   static bool ComputePythonDirectory(FileSpec &file_spec);
   static bool ComputeClangDirectory(FileSpec &file_spec);
+  static bool ComputeClangDirectory(FileSpec &lldb_shlib_spec,
+                                    FileSpec &file_spec, bool verify);
   static bool ComputeSystemPluginsDirectory(FileSpec &file_spec);
   static bool ComputeUserPluginsDirectory(FileSpec &file_spec);
 };
Index: lldb/trunk/source/Host/macosx/HostInfoMacOSX.mm
===================================================================
--- lldb/trunk/source/Host/macosx/HostInfoMacOSX.mm
+++ lldb/trunk/source/Host/macosx/HostInfoMacOSX.mm
@@ -19,6 +19,7 @@
 
 #include "llvm/ADT/SmallString.h"
 #include "llvm/Support/FileSystem.h"
+#include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
 
 // C++ Includes
@@ -226,21 +227,67 @@
 #endif
 }
 
+static bool VerifyClangPath(const llvm::Twine &clang_path) {
+  if (llvm::sys::fs::is_directory(clang_path))
+    return true;
+  Log *log = lldb_private::GetLogIfAllCategoriesSet(LIBLLDB_LOG_HOST);
+  if (log)
+    log->Printf("HostInfoMacOSX::ComputeClangDirectory(): "
+                "failed to stat clang resource directory at \"%s\"",
+                clang_path.str().c_str());
+  return false;
+}
+
 bool HostInfoMacOSX::ComputeClangDirectory(FileSpec &file_spec) {
   FileSpec lldb_file_spec;
   if (!GetLLDBPath(lldb::ePathTypeLLDBShlibDir, lldb_file_spec))
     return false;
+  return ComputeClangDirectory(lldb_file_spec, file_spec, true);
+}
 
-  std::string raw_path = lldb_file_spec.GetPath();
+bool HostInfoMacOSX::ComputeClangDirectory(FileSpec &lldb_shlib_spec,
+                                           FileSpec &file_spec, bool verify) {
+  std::string raw_path = lldb_shlib_spec.GetPath();
 
-  size_t framework_pos = raw_path.find("LLDB.framework");
-  if (framework_pos == std::string::npos)
+  auto rev_it = llvm::sys::path::rbegin(raw_path);
+  auto r_end = llvm::sys::path::rend(raw_path);
+
+  // Check for a Posix-style build of LLDB.
+  if (rev_it == r_end || *rev_it != "LLDB.framework")
     return HostInfoPosix::ComputeClangDirectory(file_spec);
-  
-  framework_pos += strlen("LLDB.framework");
-  raw_path.resize(framework_pos);
+
+  // Inside Xcode and in Xcode toolchains LLDB is always in lockstep
+  // with the Swift compiler, so it can reuse its Clang resource
+  // directory. This allows LLDB and the Swift compiler to share the
+  // same Clang module cache.
+  llvm::SmallString<256> clang_path;
+  const char *swift_clang_resource_dir = "usr/lib/swift/clang";
+  ++rev_it;
+  if (rev_it != r_end && *rev_it == "SharedFrameworks") {
+    // This is the top-level LLDB in the Xcode.app bundle.
+    raw_path.resize(rev_it - r_end);
+    llvm::sys::path::append(clang_path, raw_path,
+                            "Developer/Toolchains/XcodeDefault.xctoolchain",
+                            swift_clang_resource_dir);
+    if (!verify || VerifyClangPath(clang_path)) {
+      file_spec.SetFile(clang_path.c_str(), true);
+      return true;
+    }
+  } else if (rev_it != r_end && *rev_it == "PrivateFrameworks" &&
+             ++rev_it != r_end && ++rev_it != r_end) {
+    // This is LLDB inside an Xcode toolchain.
+    raw_path.resize(rev_it - r_end);
+    llvm::sys::path::append(clang_path, raw_path, swift_clang_resource_dir);
+    if (!verify || VerifyClangPath(clang_path)) {
+      file_spec.SetFile(clang_path.c_str(), true);
+      return true;
+    }
+  } else {
+    raw_path.resize(rev_it - r_end);
+  }
+
+  // Fall back to the Clang resource directory inside the framework.
   raw_path.append("/Resources/Clang");
-  
   file_spec.SetFile(raw_path.c_str(), true);
   return true;
 }
Index: lldb/trunk/unittests/Host/CMakeLists.txt
===================================================================
--- lldb/trunk/unittests/Host/CMakeLists.txt
+++ lldb/trunk/unittests/Host/CMakeLists.txt
@@ -22,4 +22,5 @@
   LINK_LIBS
     lldbCore
     lldbHost
+    lldbUtilityHelpers
   )
Index: lldb/trunk/unittests/Host/HostInfoTest.cpp
===================================================================
--- lldb/trunk/unittests/Host/HostInfoTest.cpp
+++ lldb/trunk/unittests/Host/HostInfoTest.cpp
@@ -8,7 +8,9 @@
 //===----------------------------------------------------------------------===//
 
 #include "lldb/Host/HostInfo.h"
+#include "lldb/Host/macosx/HostInfoMacOSX.h"
 #include "lldb/lldb-defines.h"
+#include "TestingSupport/TestUtilities.h"
 #include "gtest/gtest.h"
 
 using namespace lldb_private;
@@ -43,3 +45,41 @@
   EXPECT_EQ(HostInfo::GetAugmentedArchSpec(LLDB_ARCH_DEFAULT).GetTriple(),
             HostInfo::GetArchitecture(HostInfo::eArchKindDefault).GetTriple());
 }
+
+
+struct HostInfoMacOSXTest : public HostInfoMacOSX {
+  static std::string ComputeClangDir(std::string lldb_shlib_path,
+                                     bool verify = false) {
+    FileSpec clang_dir;
+    FileSpec lldb_shlib_spec(lldb_shlib_path, false);
+    ComputeClangDirectory(lldb_shlib_spec, clang_dir, verify);
+    return clang_dir.GetPath();
+  }
+};
+
+
+TEST_F(HostInfoTest, MacOSX) {
+  // This returns whatever the POSIX fallback returns.
+  std::string posix = "/usr/lib/liblldb.dylib";
+  EXPECT_FALSE(HostInfoMacOSXTest::ComputeClangDir(posix).empty());
+
+  std::string xcode =
+    "/Applications/Xcode.app/Contents/SharedFrameworks/LLDB.framework";
+  std::string xcode_clang =
+    "/Applications/Xcode.app/Contents/Developer/Toolchains/"
+    "XcodeDefault.xctoolchain/usr/lib/swift/clang";
+  EXPECT_EQ(HostInfoMacOSXTest::ComputeClangDir(xcode), xcode_clang);
+
+  std::string toolchain =
+      "/Applications/Xcode.app/Contents/Developer/Toolchains/"
+      "Swift-4.1-development-snapshot.xctoolchain/System/Library/"
+      "PrivateFrameworks/LLDB.framework";
+  std::string toolchain_clang =
+      "/Applications/Xcode.app/Contents/Developer/Toolchains/"
+      "Swift-4.1-development-snapshot.xctoolchain/usr/lib/swift/clang";
+  EXPECT_EQ(HostInfoMacOSXTest::ComputeClangDir(toolchain), toolchain_clang);
+
+  // Test that a bogus path is detected.
+  EXPECT_NE(HostInfoMacOSXTest::ComputeClangDir(GetInputFilePath(xcode), true),
+            HostInfoMacOSXTest::ComputeClangDir(GetInputFilePath(xcode)));
+}