[clang] [clang] Handle consteval constructors with default initialization. (PR #144970)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Came up with a different approach: I moved the check to PopExpressionEvaluationContext().  This required annotating the ExpressionEvaluationContextRecord to indicate we're handling a variable initializer.

https://github.com/llvm/llvm-project/pull/144970