[clang] [ExprConstant] Handle shift overflow the same way as other kinds of overflow (PR #99579)

[Eli Friedman via cfe-commits]

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@@ -142,7 +142,7 @@ constexpr int arb(int n) { // expected-note {{declared here}}
                expected-note {{function parameter 'n' with unknown value cannot be used in a constant expression}}
 }
 constexpr long Overflow[(1 << 30) << 2]{}; // expected-warning {{requires 34 bits to represent}} \
-                                              expected-warning {{variable length array folded to constant array as an extension}} \
+                                              expected-error {{variable length array declaration not allowed at file scope}} \
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efriedma-quic wrote:

The diagnostic shouldn't be hard to fix; it's just a matter of using the form of Evaluate() that returns notes.

For the actual behavior here... we could pass the flag to allow undefined behavior when we do this folding, I guess, but I'd rather not.  Or if we do, it should be a DefaultError diagnostic.

https://github.com/llvm/llvm-project/pull/99579