[clangd] Fix memory leak in ClangdTests.

[hokein]

Summary

createInvocationFromCommandLine sets DisableFree to true by default,
which leads memory leak in clangd. The fix is to use the BuildCompilationInvocation
to create CI with the correct options (DisableFree is false).

Fix https://bugs.llvm.org/show_bug.cgi?id=39991.

[dstenb]

With this patch applied we don't see any issues on our end. Thanks for the help!

[kadircet]

LGTM, thanks!

clang-tools-extra/trunk/unittests/clangd/TestTU.cpp

ParsedAST TestTU::build() const {
   Cmd.insert(Cmd.end(), ExtraArgs.begin(), ExtraArgs.end());
   ParseInputs Inputs;
   Inputs.CompileCommand.Filename = FullFilename;
   Inputs.CompileCommand.CommandLine = {Cmd.begin(), Cmd.end()};
   Inputs.CompileCommand.Directory = testRoot();
   Inputs.Contents = Code;
   Inputs.FS = buildTestFS({{FullFilename, Code}, {FullHeaderName, HeaderCode}});
   auto PCHs = std::make_shared<PCHContainerOperations>();
+  auto CI = buildCompilerInvocation(Inputs);
+  assert(CI && "Failed to build compilation invocation.");
   auto Preamble =
-      buildPreamble(FullFilename, *createInvocationFromCommandLine(Cmd),
+      buildPreamble(FullFilename, *CI,
                     /*OldPreamble=*/nullptr,
                     /*OldCompileCommand=*/Inputs.CompileCommand, Inputs, PCHs,
                     /*StoreInMemory=*/true, /*PreambleCallback=*/nullptr);
   auto AST = buildAST(FullFilename, createInvocationFromCommandLine(Cmd),
                       Inputs, Preamble, PCHs);
   if (!AST.hasValue()) {
     ADD_FAILURE() << "Failed to build code:\n" << Code;
     llvm_unreachable("Failed to build TestTU!");