Index: include/clang/Frontend/ASTUnit.h
===================================================================
--- include/clang/Frontend/ASTUnit.h
+++ include/clang/Frontend/ASTUnit.h
@@ -856,7 +856,8 @@
   /// \returns True if a failure occurred that causes the ASTUnit not to
   /// contain any translation-unit information, false otherwise.
   bool Reparse(std::shared_ptr<PCHContainerOperations> PCHContainerOps,
-               ArrayRef<RemappedFile> RemappedFiles = None);
+               ArrayRef<RemappedFile> RemappedFiles = None,
+               bool AllowRebuild = true);
 
   /// \brief Perform code completion at the given file, line, and
   /// column within this translation unit.
Index: lib/Frontend/ASTUnit.cpp
===================================================================
--- lib/Frontend/ASTUnit.cpp
+++ lib/Frontend/ASTUnit.cpp
@@ -2037,7 +2037,8 @@
 }
 
 bool ASTUnit::Reparse(std::shared_ptr<PCHContainerOperations> PCHContainerOps,
-                      ArrayRef<RemappedFile> RemappedFiles) {
+                      ArrayRef<RemappedFile> RemappedFiles,
+                      bool AllowRebuild) {
   if (!Invocation)
     return true;
 
@@ -2060,9 +2061,12 @@
   // If we have a preamble file lying around, or if we might try to
   // build a precompiled preamble, do so now.
   std::unique_ptr<llvm::MemoryBuffer> OverrideMainBuffer;
-  if (!getPreambleFile(this).empty() || PreambleRebuildCounter > 0)
+  if (!getPreambleFile(this).empty() || PreambleRebuildCounter > 0) {
     OverrideMainBuffer =
-        getMainBufferWithPrecompiledPreamble(PCHContainerOps, *Invocation);
+        getMainBufferWithPrecompiledPreamble(PCHContainerOps, *Invocation, AllowRebuild);
+    if (!AllowRebuild && !OverrideMainBuffer)
+      return false;
+  }
 
   // Clear out the diagnostics state.
   FileMgr.reset();