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

[Richard Smith via cfe-commits]

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@@ -200,14 +200,7 @@ namespace LongInt {
 };
 
 enum shiftof {
-    X = (1<<-29), // all-error {{expression is not an integral constant expression}} \
-                  // all-note {{negative shift count -29}}
-
-    X2 = (-1<<29), // cxx17-error {{expression is not an integral constant expression}} \
-                   // cxx17-note {{left shift of negative value -1}} \
-                   // ref-cxx17-error {{expression is not an integral constant expression}} \
-                   // ref-cxx17-note {{left shift of negative value -1}}
-
-    X3 = (1<<32) // all-error {{expression is not an integral constant expression}} \
-                 // all-note {{shift count 32 >= width of type 'int'}}
+    X = (1<<-29),
+    X2 = (-1<<29),
+    X3 = (1<<32),
----------------
zygoloid wrote:

It's concerning that we don't produce a warning by default for constant-folding this, especially given that GCC does not constant-fold bad shifts. I'm not sure it's OK that we start silently accepting this invalid code by default. I wonder if we could reasonably refuse to constant-fold if the evaluation had UB?

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