Support hierarchical document symbols

[tigersoldier]

LSP introduced hierarchical DocumentSymbol message to support hierarchical
outlines. Set hierarchicalDocumentSymbolSupport to true in the client
capabilities and handle DocumentSymbol messages in lsp-imenu.

Also fixed some document symbol bugs.

[MaskRay]

@bizzyman

hooray to see this and emacs-lsp/lsp-ui#179 I'll consider adding DocumentSymbol[] support in ccls.

It isn't very straightforward to me how to do that because declarations in C++ can be repeated (redeclarable) and there are both semantic and lexical parents.

namespace ns {
class A {
  void out_of_line();
};
void A::out_of_line() {
}
}

// redeclarable
namespace ns { int foo; }
namespace ns { int bar; }

[MaskRay]

LGTM. Thanks!

[vibhavp]

Thanks!