[clang] implement common sugared type of inst-dependent DecltypeType (#67739)

While a DecltypeType node itself is not uniqued, an instantiation
dependent DecltypeType will have a
DependentDecltypeType as an underlying type, which is uniqued.

In that case, there can be non-identical non-sugar DecltypeTypes nodes
which nonetheless represent the same type.

Fixes https://github.com/llvm/llvm-project/issues/67603