[Tooling] JSONCompilationDatabase::loadFromBuffer retains the buffer, copy it.

[sammccall]

Summary

This function doesn't seem to be used in-tree outside tests.
However clangd wants to use it soon, and having the CDB be self-contained seems
reasonable.

clang/lib/Tooling/JSONCompilationDatabase.cpp

JSONCompilationDatabase::loadFromFile(StringRef FilePath,
   return Database;
 }
 
 std::unique_ptr<JSONCompilationDatabase>
 JSONCompilationDatabase::loadFromBuffer(StringRef DatabaseString,
                                         std::string &ErrorMessage,
                                         JSONCommandLineSyntax Syntax) {
   std::unique_ptr<llvm::MemoryBuffer> DatabaseBuffer(
-      llvm::MemoryBuffer::getMemBuffer(DatabaseString));
+      llvm::MemoryBuffer::getMemBufferCopy(DatabaseString));
   std::unique_ptr<JSONCompilationDatabase> Database(
       new JSONCompilationDatabase(std::move(DatabaseBuffer), Syntax));
   if (!Database->parse(ErrorMessage))
     return nullptr;
   return Database;
 }
 
 std::vector<CompileCommand>

clang/unittests/Tooling/CompilationDatabaseTest.cpp

TEST(JSONCompilationDatabase, GetAllCompileCommands) {
   EXPECT_EQ(Command2, Commands[0].CommandLine[0]) << ErrorMessage;
   EXPECT_EQ(Directory1, Commands[1].Directory) << ErrorMessage;
   EXPECT_EQ(FileName1, Commands[1].Filename) << ErrorMessage;
   ASSERT_EQ(1u, Commands[1].CommandLine.size());
   EXPECT_EQ(Command1, Commands[1].CommandLine[0]) << ErrorMessage;
 }
 
 static CompileCommand findCompileArgsInJsonDatabase(StringRef FileName,
-                                                    StringRef JSONDatabase,
+                                                    std::string JSONDatabase,
                                                     std::string &ErrorMessage) {
   std::unique_ptr<CompilationDatabase> Database(
       JSONCompilationDatabase::loadFromBuffer(JSONDatabase, ErrorMessage,
                                               JSONCommandLineSyntax::Gnu));
   if (!Database)
     return CompileCommand();
+  // Overwrite the string to verify we're not reading from it later.
+  JSONDatabase.assign(JSONDatabase.size(), '*');
   std::vector<CompileCommand> Commands = Database->getCompileCommands(FileName);
   EXPECT_LE(Commands.size(), 1u);
   if (Commands.empty())
     return CompileCommand();
   return Commands[0];
 }
 
 TEST(JSONCompilationDatabase, ArgumentsPreferredOverCommand) {