[c-index-test] Provide capability to index module file imports and dump their input files.

This ensures the capability to index a module file using an existing ASTReader from a compiler instance or ASTUnit.

llvm-svn: 293461
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index 8064da0..f3d033c 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -619,6 +619,10 @@
     StoredDiags.emplace_back(Level, Info);
 }
 
+IntrusiveRefCntPtr<ASTReader> ASTUnit::getASTReader() const {
+  return Reader;
+}
+
 ASTMutationListener *ASTUnit::getASTMutationListener() {
   if (WriterData)
     return &WriterData->Writer;