[dcl.type.decltype] Handle splice specializations

[abhinavagarwal07]

P2996R13, applied in
#8008, specified decltype of
an unparenthesized splice expression in terms of the entity designated by its
splice-specifier.

For a splice-specialization-specifier, the contained splice-specifier
designates the template rather than the selected specialization, as specified
in basic.splice, so the wording does
not identify the intended type.

Split the cases following
expr.prim.splice and
dcl.type.splice: preserve the
existing rule for a plain splice-specifier, use the selected variable-template
specialization's type, and handle function-template overload-set cases
explicitly.