[clang][P2996] Make parameter-name queries independent of instantiation state

[Cfretz244]

Fixes #322.

The P3096 consistency walk in getParameterName (report no identifier when redeclarations disagree on a parameter name) ran over the function's redeclaration chain — but for an instantiated member, instantiating the out-of-line definition REPLACES the parameters on the same FunctionDecl without adding a redeclaration, so the walk saw exactly one name: whichever redeclaration happened to be instantiated last. The same query on the same entity answered differently depending on instantiation state, and two TUs reflecting the same entity could disagree (field shape: Eigen's DenseBase::setConstant, declared with value, defined with val).

Fix: walk the template instantiation pattern's declaration chain, which carries the in-class declaration and the out-of-line definition regardless of instantiation state. Skipped when the pattern contains a parameter pack (its parameter list does not line up index-for-index with the instantiation's; pack-substituted parameters were already filtered earlier in the function). An inconsistently-named parameter now deterministically has no identifier; consistently-named parameters keep their name.

Adds libcxx/test/std/experimental/reflection/param-name-consistency-instantiation.pass.cpp: the pre-fix flip trigger (explicit instantiation of the definition before the query) plus a consistently-named control.

Validated on a Release+assertions AArch64/macOS build of this branch: the repro flips answers at the base commit and is deterministic with the fix; clang/test/Reflection parity with base; found and exercised by differential surface-comparison of two independently generated Python binding modules over Eigen.

[zebullax]

The fix goes in the right direction, but it skips the pattern walk altogether when the function has a parameter pack anywhere, regardless of whether the parameter being queried is itself pack-derived.

template <class T> struct P {
    template <class... Args> void h(int value, Args... rest);
};
template <class T> template <class... Args>
void P<T>::h(int val, Args... rest) {}

template void P<int>::h<float>(int, float);

// The non-pack parameter "value"/"val" is inconsistently named,
// but has_identifier still returns true (reports "val"):
static_assert(!has_identifier(parameters_of(^^P<int>::h<float>)[0])); // FAILS

The pack guard (none_of(...isParameterPack())) should probably be narrowed.

[zebullax]

@Cfretz244 per above

[zebullax]

To make progress, i ll merge the above (incomplete) then push a delta.