[Clang] Fix consteval propagation for aggregates and defaulted constructors

[Clang] Fix consteval propagation for aggregates and defaulted constructors

This patch does a few things:

* Fix aggregate initialization.
  When an aggregate has an initializer that is immediate-escalating,
  the context in which it is used automatically becomes an immediate function.
  The wording does that by rpretending an aggregate initialization is itself
  an invocation which is not really how clang works, so my previous attempt
  was... wrong.

* Fix initialization of defaulted constructors with immediate escalating
  default member initializers.
  The wording was silent about that case and I did not handled it fully
  https://cplusplus.github.io/CWG/issues/2760.html

* Fix diagnostics
  In some cases clang would produce additional and unhelpful
  diagnostics by listing the invalid references to consteval
  function that appear in immediate escalating functions

Fixes https://github.com/llvm/llvm-project/issues/63742

Reviewed By: aaron.ballman, #clang-language-wg, Fznamznon

Differential Revision: https://reviews.llvm.org/D155175