[Concepts] Fix a deserialization crash.

`TemplateTypeParmDecl::hasTypeConstraint` is not a safe guard for
checking `TemplateTypeParmDecl::getTypeConstraint()` result is null.

in somecases (e.g. implicit deduction guide templates synthesized from the
constructor, immediately-declared constraint is not formed because of an error),
hasTypeConstraint returns false, and getTypeConstraint returns a nullptr.

Fix https://bugs.llvm.org/show_bug.cgi?id=46790

Differential Revision: https://reviews.llvm.org/D84455

(cherry picked from commit 73c12bd8)