Consider aggregate bases when checking if an InitListExpr is constant (#80519)

This code was correct as written prior to C++17, which allowed bases to
appear in the initializer list.

This was observable by creating non-constant aggregate initialization at
file scope in a compound literal, but since that behavior will change
soon if we implement support for dynamic initialization, I also added a
unit test for `isConstantInitializer`.

This fixes at least one part of issue #80510 .

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Co-authored-by: Aaron Ballman <aaron@aaronballman.com>