Improve error handling for Clang module imports

[aprantl]

Summary

Most notably this avoids nullptr dereferences when the import fails.

rdar://problem/48883558

[JDevlieghere]

LGTM

lldb/packages/Python/lldbsuite/test/lang/cpp/modules-import/Makefile

 LEVEL = ../../../make
 CXX_SOURCES := main.cpp
-
-CFLAGS += $(MANDATORY_MODULE_BUILD_CFLAGS)
+CFLAGS_EXTRAS = $(MANDATORY_MODULE_BUILD_CFLAGS) -I$(BUILDDIR)/include
 
 include $(LEVEL)/Makefile.rules

lldb/packages/Python/lldbsuite/test/lang/cpp/modules-import/TestCXXModulesImport.py

 """Test that importing modules in C++ works as expected."""
 
 from __future__ import print_function
 
-
-from distutils.version import StrictVersion
 import unittest2
-import os
-import time
 import lldb
-import platform
+import shutil
 
 from lldbsuite.test.decorators import *
 from lldbsuite.test.lldbtest import *
 from lldbsuite.test import lldbutil
 
 
 class CXXModulesImportTestCase(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
+    def build(self):
+        include = self.getBuildArtifact('include')
+        lldbutil.mkdir_p(include)
+        for f in ['Foo.h', 'Bar.h', 'module.modulemap']:
+            shutil.copyfile(self.getSourcePath(os.path.join('Inputs', f)),
+                            os.path.join(include, f))
+        super(CXXModulesImportTestCase, self).build()
+    
     @skipUnlessDarwin
     @skipIf(macos_version=["<", "10.12"])
     def test_expr(self):
         self.build()
-        exe = self.getBuildArtifact("a.out")
         target, process, thread, bkpt = lldbutil.run_to_source_breakpoint(
             self, 'break here', lldb.SBFileSpec('main.cpp'))
 
         self.expect("expr -l Objective-C++ -- @import Bar")
         self.expect("expr -- Bar()", substrs = ["success"])
+        self.expect("expr -l Objective-C++ -- @import THIS_MODULE_DOES_NOT_EXIST",
+                    error=True)
+
+    @skipUnlessDarwin
+    @skipIf(macos_version=["<", "10.12"])
+    def test_expr_failing_import(self):
+        self.build()
+        shutil.rmtree(self.getBuildArtifact('include'))
+        target, process, thread, bkpt = lldbutil.run_to_source_breakpoint(
+            self, 'break here', lldb.SBFileSpec('main.cpp'))
+
+        self.expect("expr -l Objective-C++ -- @import Bar", error=True)

lldb/source/Plugins/ExpressionParser/Clang/ClangModulesDeclVendor.cpp

if (module.search_path) {
     auto sysroot_begin = llvm::sys::path::begin(module.sysroot.GetStringRef());
     auto sysroot_end = llvm::sys::path::end(module.sysroot.GetStringRef());
     // FIXME: Use C++14 std::equal(it, it, it, it) variant once it's available.
     bool is_system_module = (std::distance(path_begin, path_end) >=
                              std::distance(sysroot_begin, sysroot_end)) &&
                             std::equal(sysroot_begin, sysroot_end, path_begin);
     // No need to inject search paths to modules in the sysroot.
     if (!is_system_module) {
+      auto error = [&]() {
+        error_stream.Printf("error: No module map file in %s\n",
+                            module.search_path.AsCString());
+        return false;
+      };
+
       bool is_system = true;
       bool is_framework = false;
       auto *dir =
           HS.getFileMgr().getDirectory(module.search_path.GetStringRef());
+      if (!dir)
+        return error();
       auto *file = HS.lookupModuleMapFile(dir, is_framework);
+      if (!file)
+        return error();
       if (!HS.loadModuleMapFile(file, is_system))
-        error_stream.Printf("error: No module map file in %s\n",
-                            module.search_path.AsCString());
-      return false;
+        return error();
     }
   }
   if (!HS.lookupModule(module.path.front().GetStringRef())) {
     error_stream.Printf("error: Header search couldn't locate module %s\n",
                         module.path.front().AsCString());
     return false;
   }