[clang] Use decltype((E)) for compound requirement type constraint

See PR45088.

Compound requirement type constraints were using decltype(E) instead of
decltype((E)), as per `[expr.prim.req]p1.3.3`.

Since neither instantiation nor type dependence should matter for
the constraints, this uses an approach where a `decltype` type is not built,
and just the canonical type of the expression after template instantiation
is used on the requirement.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D98160