[Sema] Fix assertion failure when instantiating requires expression

Fixes #54629.
The crash is is caused by the double template instantiation.
See the added test. Here is what happens:
- Template arguments for the partial specialization get instantiated.
- This causes instantitation into the corrensponding requires
  expression.
- `TemplateInsantiator` correctly handles instantiation of parameters
  inside `RequiresExprBody` and instantiates the constraint expression
  inside the `NestedRequirement`.
- To build the substituted `NestedRequirement`, `TemplateInsantiator`
  calls `Sema::BuildNestedRequirement` calls
  `CheckConstraintSatisfaction`, which results in another template
  instantiation (with empty template arguments). This seem to be an
  implementation detail to handle constraint satisfaction and is not
  required by the standard.
- The recursive template instantiation tries to find the parameter
  inside `RequiresExprBody` and fails with the corresponding assertion.

Note that this only happens as both instantiations happen with the class
partial template specialization set as `Sema.CurContext`, which is
considered a dependent `DeclContext`.

To fix the assertion, avoid doing the recursive template instantiation
and instead evaluate resulting expressions in-place.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D127487