[concepts] Extract function template pack arguments from the current instantiation if possible (#80594)

Before the constraint substitution, we employ
`getTemplateInstantiationArgs`, which in turn attempts to inspect
`TemplateArgument`s from the function template. For parameter packs from
their parent contexts, we used to extract the arguments from the
specialization type, in which could result in non-canonical argument
types e.g. `PackExpansionType`.

This may break the contract that, during a tree transformation, in
`TreeTransform::TryExpandParameterPacks`, the corresponding
`TemplateArgument`s for an `UnexpandedParameterPack` are expected to be
of `Pack` kinds if we're expanding template parameters.

Fixes https://github.com/llvm/llvm-project/issues/72557.

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