[ClangImporter] Stop abusing a Clang API to avoid a use-after-free. (#11377)

In order to allow LLDB to debug an app as it was written (in theory),
we stuff a semi-preprocessed version of the bridging header into the
generated swiftmodule file, so that we can fall back on it if the
original bridging header is deleted. The specific functionality we
want is in Clang's -rewrite-includes option, but we need to write to
an in-memory buffer instead of a file.

The existing code took the meat of clang::RewriteIncludesAction and
recreated it manually. We got away with this in the past, but upstream
Clang has changed how clang::RewriteIncludesAction works, and now what
we were doing before results in the (unused) file output stream being
deallocated before the (also unused) buffering stream wrapped around
it. Rather than continue trying to half-use an existing Clang
FrontendAction, just make a new one that does the thing we want and
nothing else.

I'd like to revisit this whole idea in the future -- it doesn't
actually preserve debuggability in the case where the bridging header
may have changed, because we don't check for modifications in all the
things it includes. But for now this should preserve the existing
functionality.

rdar://problem/33693128

Author: jrose-apple

lib/ClangImporter/ClangImporter.cpp

@@ -1289,13 +1289,23 @@ std::string ClangImporter::getBridgingHeaderContents(StringRef headerPath,
 
   std::string result;
   bool success = llvm::CrashRecoveryContext().RunSafelyOnThread([&] {
-    clang::RewriteIncludesAction action;
-    action.BeginSourceFile(rewriteInstance,
-                           invocation->getFrontendOpts().Inputs.front());
+    // A much simpler version of clang::RewriteIncludesAction that lets us
+    // write to an in-memory buffer.
+    class RewriteIncludesAction : public clang::PreprocessorFrontendAction {
+      raw_ostream &OS;
+
+      void ExecuteAction() override {
+        clang::CompilerInstance &compiler = getCompilerInstance();
+        clang::RewriteIncludesInInput(compiler.getPreprocessor(), &OS,
+                                      compiler.getPreprocessorOutputOpts());
+      }
+    public:
+      explicit RewriteIncludesAction(raw_ostream &os) : OS(os) {}
+    };
+
     llvm::raw_string_ostream os(result);
-    clang::RewriteIncludesInInput(rewriteInstance.getPreprocessor(), &os,
-                                  rewriteInstance.getPreprocessorOutputOpts());
-    action.EndSourceFile();
+    RewriteIncludesAction action(os);
+    rewriteInstance.ExecuteAction(action);
   });
 
   success |= !rewriteInstance.getDiagnostics().hasErrorOccurred();