[clangd] Do not offer extraction to variable for decl init expression (#69477)

That would turn:
  int x = f() + 1;
into:
  auto placeholder = f() + 1;
  int x = placeholder;
which makes little sense and is clearly not intended, as stated
explicitly by a comment in eligibleForExtraction(). It appears that the
declaration case was simply forgotten (the assignment case was already
implemented).