[Concepts] Fix ConceptSpecializationExpr profiling crash

ConceptSpecializationExprs (CSEs) were being created with nullptr
TemplateArgsAsWritten during TemplateTemplateParmDecl canonicalization, and
we were relying on them during profiling which caused sporadic crashes
in test/CXX/.../temp.arg.template/p3-2a.cpp introduced in D44352.

Change profiling of CSEs to instead rely on the actual converted template
arguments and concept named.