[clangd] Fix type printing in the presence of qualifiers

When printing QualType with qualifiers like "const", or pointing to an
elaborated type, we would print garbage like:
  std::const std::vector<int>&
with the initial std:: being calculated correctly, but inserted in the
wrong place and the second std:: not removed (due to elaborated type).

This affected, among others, ExtractFunction and ExpandAuto tweaks.

This change introduces a new callback to PrintingPolicy, which allows us
to influence the printing of namespace qualifiers. In the future, the
same callback can be used to improve handling of "using namespace"
directives as well.

Fixes:
  https://github.com/clangd/clangd/issues/640 (ExtractFunction)
  https://github.com/clangd/clangd/issues/264 (ExpandAuto)
  First point of https://github.com/clangd/clangd/issues/524

Differential Revision: https://reviews.llvm.org/D94259