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

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.

Differential Revision: https://reviews.llvm.org/D92646

Author: sam-mccall

clang/lib/Tooling/JSONCompilationDatabase.cpp

@@ -217,7 +217,7 @@ 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))

clang/unittests/Tooling/CompilationDatabaseTest.cpp

@@ -172,13 +172,15 @@ TEST(JSONCompilationDatabase, GetAllCompileCommands) {
 }
 
 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())