[Clang] Handle consteval expression in array bounds expressions (#66222)

The bounds of a c++ array is a _constant-expression_. And in C++ it is
also a constant expression.

But we also support VLAs, ie arrays with non-constant bounds.

We need to take care to handle the case of a consteval function (which
are specified to be only immediately called in non-constant contexts)
that appear in arrays bounds.

This introduces `Sema::isAlwayConstantEvaluatedContext`, and a flag in
ExpressionEvaluationContextRecord, such that immediate functions in
array bounds are always immediately invoked.

Sema had both `isConstantEvaluatedContext` and
`isConstantEvaluated`, so I took the opportunity to cleanup that.

The change in `TimeProfilerTest.cpp` is an unfortunate manifestation of
the problem that #66203 seeks to address.

Fixes #65520