[clang] Fix visitation of ConceptSpecializationExpr in constrained-parameter

Summary: RecursiveASTVisitor needs to traverse TypeConstraint::ImmediatelyDeclaredConstraint

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D84136