clang assert on invalid with is_constant_evaluated in template argument

[efriedma-quic]

template<typename T, int I = 1/!__builtin_is_constant_evaluated()> class X;
void f(X<int, 1>* x) { x->a; }

templateArgumentExpressionsEqual() uses isIntegerConstantExpr(), then EvaluateKnownConstInt()... but they don't agree whether the InConstantContext should be set.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Eli Friedman (efriedma-quic)

``` template<typename T, int I = 1/!__builtin_is_constant_evaluated()> class X; void f(X<int, 1>* x) { x->a; } ```

templateArgumentExpressionsEqual() uses isIntegerConstantExpr(), then EvaluateKnownConstInt()... but they don't agree whether the InConstantContext should be set.

[Fznamznon]

Confirmed https://godbolt.org/z/1Gv3M8W63 .