[Frontend] Honor UserFilesAreVolatile flag getting file buffer in ASTUnit

Do not memory map the main file if the flag UserFilesAreVolatile is set to true
in ASTUnit when calling FileSystem::getBufferForFile.

Differential Revision: https://reviews.llvm.org/D47460

llvm-svn: 334070
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index e0e8e8b..d7bbf6f 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -156,7 +156,8 @@
 static std::unique_ptr<llvm::MemoryBuffer>
 getBufferForFileHandlingRemapping(const CompilerInvocation &Invocation,
                                   vfs::FileSystem *VFS,
-                                  StringRef FilePath) {
+                                  StringRef FilePath,
+                                  bool isVolatile) {
   const auto &PreprocessorOpts = Invocation.getPreprocessorOpts();
 
   // Try to determine if the main file has been remapped, either from the
@@ -176,7 +177,7 @@
         llvm::sys::fs::UniqueID MID = MPathStatus->getUniqueID();
         if (MainFileID == MID) {
           // We found a remapping. Try to load the resulting, remapped source.
-          BufferOwner = valueOrNull(VFS->getBufferForFile(RF.second));
+          BufferOwner = valueOrNull(VFS->getBufferForFile(RF.second, -1, true, isVolatile));
           if (!BufferOwner)
             return nullptr;
         }
@@ -201,7 +202,7 @@
 
   // If the main source file was not remapped, load it now.
   if (!Buffer && !BufferOwner) {
-    BufferOwner = valueOrNull(VFS->getBufferForFile(FilePath));
+    BufferOwner = valueOrNull(VFS->getBufferForFile(FilePath, -1, true, isVolatile));
     if (!BufferOwner)
       return nullptr;
   }
@@ -707,7 +708,7 @@
 std::unique_ptr<llvm::MemoryBuffer>
 ASTUnit::getBufferForFile(StringRef Filename, std::string *ErrorStr) {
   assert(FileMgr);
-  auto Buffer = FileMgr->getBufferForFile(Filename);
+  auto Buffer = FileMgr->getBufferForFile(Filename, UserFilesAreVolatile);
   if (Buffer)
     return std::move(*Buffer);
   if (ErrorStr)
@@ -1278,7 +1279,7 @@
       PreambleInvocationIn.getFrontendOpts().Inputs[0].getFile();
   std::unique_ptr<llvm::MemoryBuffer> MainFileBuffer =
       getBufferForFileHandlingRemapping(PreambleInvocationIn, VFS.get(),
-                                        MainFilePath);
+                                        MainFilePath, UserFilesAreVolatile);
   if (!MainFileBuffer)
     return nullptr;