This patch factors out the completion logic for individual clang::Type's
into their own helper functions.
During the process I cleaned up a few assumptions (e.g., unnecessary
if-guards that could be asserts because these conditions are guaranteed
by the `clang::Type::TypeClass` switch in `GetCompleteQualType`).
This is mainly motivated by the type-completion rework proposed in
https://github.com/llvm/llvm-project/pull/95100.
f2d215f572