[Tooling] Don't mmap the JSONCompilationDatabase, it's not correct for long-lived processes.

lib/Tooling/JSONCompilationDatabase.cpp

 
 } // namespace tooling
 } // namespace clang
 
 std::unique_ptr<JSONCompilationDatabase>
 JSONCompilationDatabase::loadFromFile(StringRef FilePath,
                                       std::string &ErrorMessage,
                                       JSONCommandLineSyntax Syntax) {
+  // Don't mmap: if we're a long-lived process, the build system may overwrite.
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> DatabaseBuffer =
-      llvm::MemoryBuffer::getFile(FilePath);
+      llvm::MemoryBuffer::getFile(FilePath, /*FileSize=*/-1,
+                                  /*RequiresNullTerminator=*/true,
+                                  /*IsVolatile=*/true);
   if (std::error_code Result = DatabaseBuffer.getError()) {
     ErrorMessage = "Error while opening JSON database: " + Result.message();
     return nullptr;
   }
   std::unique_ptr<JSONCompilationDatabase> Database(
       new JSONCompilationDatabase(std::move(*DatabaseBuffer), Syntax));
   if (!Database->parse(ErrorMessage))
     return nullptr;