[Clang] Realize generic lambda call operators are dependent sooner

When parsing a trailing return type / noexcept / constraint
of a generic lambda, we need to know that we are in a dependent
context. We currently don't because we only create a TemplateDecl
for the call operator one its fully parsed.

This patch attach a template decl to the call operator as soon
as the parameter declaration clause is parsed - the point at which
we  have collected all template parameters

Fixes #64689

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D159358