[clang][cas] Fix crash on invalid with IncludeTreeBuilder

If an error occurs in FrontendAction::BeginSourceFile after
IncludeTreeActionController::initializeModuleBuild is called (via
BeginInvocation), the builder is left in an inconsistent state because
there is no call to finalizeModuleBuild (via HandleTranslationUnit), so
we should exit early without attempting to create an include tree for
the broken module. Fixes a crash on invalid seen with a missing module
header, but there could be other similar cases.

rdar://151878898
(cherry picked from commit 5d380b9)

Author: benlangmuir

clang/lib/Tooling/DependencyScanning/IncludeTreeActionController.cpp

@@ -386,6 +386,13 @@ Error IncludeTreeActionController::finalizeModuleBuild(
       ModuleScanInstance.getInvocation().getLangOpts(),
       ModuleScanInstance.getInvocation().getCodeGenOpts());
   auto Builder = BuilderStack.pop_back_val();
+
+  // If there was an error, bail out early. The state of `Builder` may be
+  // inconsistent since there is no guarantee that exitedInclude or
+  // finalizeModuleBuild have been called for all imports.
+  if (ModuleScanInstance.getDiagnostics().hasUnrecoverableErrorOccurred())
+    return Error::success(); // Already reported.
+
   auto Tree = Builder->finishIncludeTree(ModuleScanInstance,
                                          ModuleScanInstance.getInvocation());
   if (!Tree)

clang/test/ClangScanDeps/modules-include-tree-missing-header.c

@@ -0,0 +1,28 @@
+// Tests that a missing header in a module that is itself imported by another
+// module does not crash/assert in the IncludeTreeBuilder.
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// RUN: not clang-scan-deps -format experimental-include-tree-full -cas-path %t/cas -- %clang -fmodules -fmodules-cache-path=%t/cache -c %t/tu0.m -I%t
+// RUN: not clang-scan-deps -format experimental-include-tree-full -cas-path %t/cas -- %clang -fmodules -fmodules-cache-path=%t/cache -c %t/tu1.m -I%t
+
+//--- module.modulemap
+module MissingH {
+  header "missing.h"
+}
+
+module Importer {
+  header "importer.h"
+}
+
+//--- not-missing.h
+
+//--- importer.h
+@import MissingH;
+
+//--- tu0.m
+@import MissingH;
+
+//--- tu1.m
+@import Importer;