[clangd] Fix memory leak in ClangdTests.

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.

Reviewers: kadircet

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D55702

llvm-svn: 349145