[clangd] Fix leak sanitizers warnings in clangd

The commit includes two changes:
1. Set DisableFree to false when building the ParsedAST.
   This is sane default, since clangd never wants to leak the AST.
2. Make sure CompilerInstance created in code completion is passed to
   the FrontendAction::BeginSourceFile call.
   We have to do this to make sure the memory buffers of remapped
   files are properly freed.

Our tests do not produce any warnings under asan anymore.
The changes are mostly trivial, just moving the code around. So
sending without review.

llvm-svn: 333370